I found many interesting comments on an MR post about the Peace Corps. I got the impression that the Peace Corps doesn't materially help foreigners much at all, but that it does give them a better impression of the U.S. For example, I followed a link to this:

The substantive work I did wasn't all that useful... However, and this is the big kicker, while I don't know how useful PC is generally to the people we're trying to help, I'm very certain that it is incredibly, incredibly helpful to the US's image abroad. Samoa gets a lot of foreign aid, because it's both poor and peaceful. Japan, Australia, New Zealand, and China all had programs costing tens of millions of dollars in road building, harbor dredging -- expensive, useful stuff. The US gave very little aid comparatively, but a whole lot of Volunteers, and we got the credit for everything.

On the other hand, the above provoked a comment claiming that the Peace Corps is not so loved in the Dominican Republic.