Many people with dementia find themselves routinely having things done 'for' them or 'to' them. When a person shares something about their past and another person shows interest or enjoyment, it is a wonderful opportunity for that person to feel that they are the one who is giving something to another human being, rather than always being the one who is receiving or listening.
Talking about the past can also bring up happy memories and good feelings, and this can be wonderful in itself, but particularly if a person is finding life difficult.
It is also the case that reminiscence can sometimes provoke painful memories. Emotional reactions are not necessarily a bad thing, but we need to respond sensitively.