Re: Taliban cut off heads of 17 people for having music at party 
Islamic law doesn’t prevent adultery from occurring, instead it prevents certain behaviors from becoming publicly acceptable.
As long as Satan exists, mankind including Muslims will be tempted to sin, from the most minor of sins to the most serious of sins, and many will always give in to temptation.
Shariah law, and the high standards of evidence that it entails, prevent undesirable behaviors from becoming publicly accepted.
Yes, even in the most liberal of modern societies, people are not going to go about (in general) having adulterous sex in public, where 4-5 adult witnesses will see it. However, it is increasingly common for people to openly admit to marital infidelity, such as to friends. This is the road to societies where adultery is openly winked it, such as France or Italy, where it is macho to have a mistress.
The purpose of such punishments is to prevent Muslim societies from such behavior patterns.