This has always bugged me about volunteer work. Don't get me wrong, there are places that need volunteers, but those places are local. If you want to volunteer to hang out with retired folks in a local nursing home, you are a hero. If you travel to Nigeria and are not a highly skilled professional going to ply your rare craft, you are a shit. And by rare craft, I mean that you have years of first hand experience with it, not that you get a 10 hour course in something. If you are in college or just out of college, let me assure you that you have absolutely nothing to contribute and are actively destructive. Take all that money you and the charity was going to spend to send your pasty ass out there and divide it into two piles. In one pile, put money to help the locals hire local labor for vastly less than the price of your ticket and accommodations. In the other pile, put your vacation money and go be a tourist somewhere. You will do more good and still get to go have your adventure.
Developing nations don't need "free" labor. They can hire a dozen strong men for a month for the cost of shipping one fat American over for a week. They might need some very specific skilled labor. They need aid to develop industry and jobs to support themselves.