I found the OP's question interesting, if unusual, but perhaps that's because I've just had the opportunity to volunteer with so many different things over the past 14 years or so.
Perhaps my most substantial contribution (and most personally rewarding) is the time I've put into volunteering with a local kids' summer camp. It's been 13 years now that I've gone to camp as a counsellor for 2-3 weeks per year (and if I was employed at the time, I'd call that my vacation time). I'm currently in my 6th (and last) year of a term on the Board of Directors, as Vice President of the organization
Apart from the usual counselling and spending time with the kids at camp, I also help with all the tech services such as taking digital pictures throughout the week, compiling them into slide shows, maintaining the camp website, brochures, newsletters, and so on.
I've also spent, over the years, what probably amounts to thousands of dollars in material donations as well as my own airfare to attend various conferences all over the USA on behalf of camp (and its national Association).
The "why" is an interesting question. I suppose just because.. it's fun! I love what I do with camp, it's been an opportunity to exercise and improve my skills, and do something very different from my day job (software engineer). Also, I've been with camp long enough to see 8-year-old kids grow and mature into responsible young adults who now also come back as staff members, and to have had a part in being a role model is somehow indescribable. My closest friends (and my girlfriend

) are all from camp.
Volunteer because you love doing it, not because it's hours to be counted or for any other reason.