the peace corps generally uses volunteer labor to increase development. development (inclusion into the western model of the world) has some short term benefits and many short and long term detriments.
as far as i know, anarchists don't like charity not because it decreases suffering (?!), but because it continues a power relationship that is inappropriate, that assumes that people cannot solve their own problems, that assumes that outside influences know better what a people need than the "charity-ed" do, etc.