What were the benefits of joining the Delian League?

Answer 1

For the nearly 200 city-states exposed to Persian domination, protection (at a price). This benefit ceased when the Persians agreed to a peace which kept them out of the area, but Athens continued to levy the price and spend the money on a fleet not against the Persians but to continue collecting the money, by force if necessary, and spent it on its own city beautification and public service.