and 2 other important reasons:
1. to pass it all on. To those who currently don't consider it important, but who might, one day. My daughter recently spent a few hours with my Mum listening to her life story - got quite tearful at times - and pronounced how "I need to get this all down, so your story is never forgotten!". (Hello, had she not realised what I've been doing for the past 4 years??!!

2. Selfish reason: recording my Mum & Dad telling all about their lives, especially as children during WW2 means that one day in the future, when they are no longer here, I can still turn on a machine of some sort, and listen to their voices. (I mentioned this in response to another similar thread recently, so apologies for the repeated comments).
These memories are precious, maybe in a larger social historical context, but certainly in our own families.