For them
When I started doing genealogy it was to discover my roots. And yes I'd read the book Roots and I'm not sure that started me, as I was already interested, but it certainly helped.
As the years went by I became more interested in leaving a legacy. I didn't know my ancestors beyond my grandparents. I've found them and I want my children, grandchildren, and the rest of my descendants to have the knowledge that I've found.
That legacy is what interested me in obtaining my Certified Genealogist title. I wanted to have assurances that I was on the right track and creating solid genealogical reports. I've now found fun in doing research for others, but the main goal was to create solid research for my genealogy.