Each state has a different format for everything, be it census records, war records, birth certificates, marriage licenses, and death certificates. I love seeing the different kinds and what the different states asked on each records - nothing was unified 100 years ago. For example, not all states asked for the soldier's mother's maiden name on their draft cards, but some did. Same with birth certificates and death records - it was all up to the state what was recorded.
I'm also using it as practice to learn about identifying different documents. Eventually (not tomorrow or anywhere close), I intend to do the genealogy thing as a part-time profession. I have a journalism degree, a background in history (both things that lend themselves to research), and I'm naturally curious. I love helping friends find their roots - I've done it for several friends, including one memorable family history hunt for a good friend of mine that I will never blog about but it taught me alot about research, skeletons, and life in general. Someday I want to get certified and do it for real, and indexing helps me familiarize myself with all the records I someday have to know.
Helping others is the main factor in my doing this. I hope that some document I index, somewhere along the way, helps someone fill a hold in their own family stories. I'm grateful for every single person who has helped document the details of my ancestors - I consider this paying it forward.