Two major reasons, really. One is to manipulate you by manipulating your worldview. People's sense of self is heavily tied in to their memes--the memories they have created and their interpretation of them. With your memories altered, your opinions of your life, your whole personality, can be shifted in the way that the alterer wants.

The other is to cover their own tracks, so you don't suspect them of manipulating you or doing some other wrong.

[Sadness crosses her features.] Whatever the case, I really don't think this was done with your consent.