Mufti Ebrahim Desai explains it here:
Question:. When a husband divorces his wife three times, she is already traumatized, why does Shariah punish her again by subjecting her to halalah? Why isn't a simple nikah sufficient without consummation? Why should the divorcee be dirtied by another man? I personally won't return to such a woman. Doesn't the process of halalah also create temptations of other men in the divorcee?
Answer.: The concept of halalah, where the divorcee of three times (Talaq-e-Mugallazah) is required to marry another man and have the marriage consummated as a precondition for her to revert to her husband is expressly mentioned in the Quran (Baqarah: 230) and many Ahadith (Bukhari, vol. 2 - pg. 731 H.M. Saeed). Allah Ta'ala is Al-Hakeem (The All-Wise). Every decree of Allah is filled with wisdom. The full depth of the wisdom of Allah Ta'ala can never be fathomed by all the most intelligent people of the world. We as the slaves of Allah Ta'ala are duty bound to express submission to Allah. He is our Master and we are His slaves. We have to simply obey the orders of Allah whether we understand them or not. Allah out of infinite mercy has also granted the understanding of deen to the Ulama. That understanding creates a sense of fulfillment in expressing servitude to Allah. The practice of halalah is the express order of Allah. This was also advised by Rasulullah (sallallahu alayhi wasallam) to the wife of Rifa'ah Al-Qurazi who was previously married to Abdul Rahman Ibn Zabeer. This incident is recorded in almost every book of Hadith and several places in Bukhari. Let us briefly discuss the wisdom of this decree of Allah. Shariah has granted a lofty position to a woman as a daughter, as a wife, as a sister and as a mother. She ascends on the throne of nobility as she passes the different phases of a woman. My focus here is only on her position as a wife and maintaining the marriage bond.
It is mentioned in Bukhari that at the time of Jaahiliyah a man could divorce his wife for any number of times and take her back as he wished to. Any noble woman would understand the trauma of this practice. She can never be focused in life. There is absolutely no dignity or honor for her if she is repeatedly divorced. Her position as a wife is always in suspense. She was no more than a cheap commodity, in fact worse. The reasons for that are clear as she is a human being with feelings equal to a man. Shariah has restored her with a right of dignity and restricted the unlimited number of divorces. If a man wants her as a wife, he must understand the commitment of marriage and the respect of his wife. He knows ahead of time that the limit of divorce is three. Shariah has advised him that divorce is the most abhorred thing. Rasulullah (sallallahu alayhi wasallam) said, Verily the most abhorred among permissible things is divorce. The husband must exercise restraint and tolerate his wife just as she too, tolerates him.
If he divorces her expressly one or two times (Talaq-e-Raji) in one sitting or in different sittings then he can unilaterally take her back within the period of iddat. He cannot take her back thereafter without her consent. This is also the ruling if an indicative divorce (Talaq-e-Ba'in) was issued. It is now her independent right. She must decide whether she wants to live with the man that divorced her and abused her. Once he gives her three divorces, he just cannot have her back. Now it is not even her right to decide to go back to her ex-husband. Shariah steps in to protect her dignity and honor and stop abuse against her. A woman by nature gives in more often and easily than a man. It is possible she may simply subject herself to her abusive husband. Shariah considers her nature and now throws a strong armour around her. She now is the sole right of the Shariah. Now nobody owns her. Her abusive husband needs to be treated with contempt. Any noble man who knows the consequences of three divorces and the process to have his wife back will never divorce his wife. He will respect her and tolerate her. The purpose of halalah is clearly misunderstood. It is incorrectly regarded as an unjust punishment to the divorcee. In fact, the institution of halalah is to protect a woman against the abuse of divorce. If a man cannot respect his wife, he is not worthy of respect. He must be taught the hard way. This does not mean that halalah is a punishment to the woman. Shariah does not impose on her to remarry. It is her right. She can choose not to re-marry. If she does marry, it should not be to merely fulfill a prerequisite to revert to her ex-husband. It is to be a marriage of honor and dignity, a permanent and normal marriage with all its rights. If this marriage happens to be unsuccessful, then again she has the right to marry her ex-husband. She does not have to if she chooses not to remarry him. It is clear from the above explanation that the concept of halalah is:
1. To protect the wife from the abuse of her husband.
2. The husband should understand the implications of three divorces and exercise restraint.
3. If he does not contain himself, he cannot be given an unlimited right of divorce.
The above explanation and subsequent points fulfil you. Now, reverting to your points;
1.You enquire why must she consummate her marriage with her new husband. Our response is why not. Why did she re-marry? Just to legitimize her return? She should not enter into a marriage with that intention. If she does so, what remedy is there for a woman who does not accept the dignity granted to her by Shariah? She is subjecting herself to a process for her abusive ex-husband.
2.You state you will never return to a woman dirtied by another man. If you feel like that, you can be a perfect husband and never divorce your wife for her to be "dirtied" by another man. If you do divorce her, why would you be worried about her being dirtied for you? You did worse to her by your abuse to her than her merely being dirtied. An abusive husband needs the psychological punishment of halalah. If the ex-husband feels she is dirtied, he doesn't have to take her back if he doesn't want to. Nobody forces him to do so. It is his choice. He must decide. If he takes her back, why call her dirty?
3.You enquire that her sleeping with another man creates more temptations for her. That is correct. The husband must realize all this ahead of time and exercise control. A chaste woman will only fulfil herself in a legitimate avenue. She will look after her chastity and respect. We are discussing the preservation of the dignity and honor of an Allah fearing chaste woman, not just any woman.
Mufti Ebrahim Desai
Subhan Allah - i believe this part explains it all beautifully:
Once he gives her three divorces, he just cannot have her back. Now it is not even her right to decide to go back to her ex-husband. Shariah steps in to protect her dignity and honor and stop abuse against her. A woman by nature gives in more often and easily than a man. It is possible she may simply subject herself to her abusive husband. Shariah considers her nature and now throws a strong armour around her. She now is the sole right of the Shariah. Now nobody owns her. Her abusive husband needs to be treated with contempt. Any noble man who knows the consequences of three divorces and the process to have his wife back will never divorce his wife. He will respect her and tolerate her.
Allah SWT has created laws for the benefit of mankind. this law is to protect the woman.
halalah SHOULD NOT be done with intention of reuniting with original husband.