I can only tell you why I do it, but it's for more than one reason.
Scrapbooks make great external hard drives. I have never had a great memory. Little details from the past are always replaced in my brain with current events. Scrapbooks help me dig up those older memories. They help my children see things in their past, from when they were too young to remember, and me to remember them when they were young.
I have always scrapbooked, starting in my teenage years with a true scrapbook. A bound book of papers in which I stuck all the little scraps of paper that meant something to me. When I started travelling, I added pictures to those scraps of paper (ticket stubs, flyers, napkins). When I am no longer able to go and do (at 90, 95 years old!), I want to look back and remember that I DID!
I'm crafty. I like to create, to make things. I'm no expert creator, and sticking to paper keeps it cheap. But I've always loved the paper side of it. When I was about 7, I got a box of goodies for Christmas. It was a big (for a 7 year old) cardboard box, filled with construction paper, glue, markers, a box of 64 crayons (with sharpener), pom-poms, magnets and pipe cleaners. My mother has always thought it was one of the worst gifts they ever gave me as a kid; I've always considered it one of the best.
I think it's innate that we want to create or build something. Quilts, cakes, gardens, paintings, jigsaw puzzles, whatever. I love that my creating is something I want to keep and that has a purpose to me. Others may look and the great ones actually admire my work, but ultimately, I scrap book for me. I scrap to tell myself the story of my life.
So. Why do you scrapbook?