Freud showed that some psychotic disorders could be fixed if certain ‘repressed’ memories were drawn into the ‘concious’ part of the brain.
People don’t necessarily actively try to forget things in their past. Sometimes they do, but everything in the past fades away as time goes by. New memories butt in for room amongst the old ones, and sit more towards the ‘front’ of ‘conciousness’. Current events are most commonly the most important at any given moment, so they are given prime memory real estate, while old ones are filed away.
Every even in ones past is essentially what one is in the present. If any of those past things had happened differently, the present person would be different. It may be such a slight difference as to be unnoticable, or it may be so large that it would seem impossible the two people could be the same one biologically, depending on the significance of the effects of the event on that person. The human psyche has a tendancy to want to know who it is. Who it was is very important to understanding who it is. As everything in the past has an effect on the present, an understanding of that past helps understand who that present person is. The more significant the event, the greater an effect it had on the present, and thus the more important its understanding helps understand the present.
So unearthing hidden memories of significance helps one understand who one is, allowing one to be more comfortable with oneself.