The main goal of an MPA—and I'll follow the IUCN definition—is conservation, so the benefits I see are mostly conservation, which include fish.
Typically a lot of studies have looked at inside MPAs versus outside MPAs, and before and after, and shown that fish get bigger and the diversity of fish and species is larger when you create an MPA. That has been well documented. I believe Dr. Ban came and gave a number of papers and evidence on these effects.
I am one of the proponents who thinks that MPAs are actually very good for the community—mostly the small-scale fisheries—because they are a very good way to protect the land or the piece of water that is close to your community. In short, if you screw it up in front of your community and you're a small-scale fishery, you don't have any plan B.
MPAs really help you sustain the system locally, unlike the industrial fishing that has more ability to move.