I think that the pendant is what keeps Walter's memories, and the longer he goes without the pendent, the more memories he will recover, that's why his mother tells him that he has to keep the pendant always, besides, he starts to remenber small things when you lose the pendant for the first time, as for what it will be used for I don't know it's probably the key to something (along with Mark's watch and Lars' pendant) probably that something has to do with the the fact that Walter doesn't have childhood memories, I think there's something sealed in Walter's memories, something that can't get out, that's why Walter's friends don't mention the incident to him.