I have a couple of reasons.
First, I have an unreasonable fear that I will die before my children are grown. I want them to know without a shadow of a doubt how much their parents love them. To that end I don't just scrap photos, but thoughts, messages and lessons/advice for them. No one else can tell them the feelings in my heart. No one else knows the things I want to teach them. Even their Dad, try though he might. I'm the best one to do that, so I do it now, through my scrapbooking. My girls get it. At 3 yrs old I asked my eldest how she knows mummy loves her. She said "You scrapbook".
All too soon my girls will be grown and gone from our home. I use the time I have to say what I can, and record it on a scrapbook page just in case I'm not here one day when they need mum's advice. And just maybe when they're ornery teenagers who don't want to listen to mum, maybe they'll still read their scrapbooks and hear my messages anyway.
Secondly, I scrap for a creative release. It's the one hobby (besides reading) that I do for myself, and I enjoy it. I like recording family history for my immediate and extended family.