It’s actually more than doing a good deed (which would be like reading a fateha or feeding the poor in his name). It’s because she believes that the deceased wait to be acknowledged by their family. On the rare Thursday she’s unable to read the surah, she feels like he was disappointed because he was waiting for her and she didn’t acknowledge him.
It’s kind of weird to explain. It’s just that I think the deceased still “feel” after they’ve died and that they can listen and know what their loved ones are doing.