Certain deities correspond to certain areas with some degree of overlap, if you wanted things to go according to plan they needed to be appeased, you might not like them (Ares wasn't thought of fondly by the Greeks) but they all needed to be paid off nevertheless. Say you were a ship's captain about to leave port, obviously you are going to give an offering to Poseidon an offering as he's god of the seas, but because you want a favourable wind, you're going to give one to Zeus too, because if you neglected either one you'd face difficulty from that area even if you were good with other one(s).

If you want, there's a series of blog posts from a lecturer at the university of North Carolina that goes into the nuts and bolts of polytheism and how dealing with multiple gods worked.