The simple answer is that we don't know. The true nature of a god is obviously beyond our understanding: the discrepancy of scale is just too great. One can find a pair of gods who seem to have similar personalities or fields of interest, but that's just the tip of the iceberg.
Naturally, we all have similar needs and so look to gods to tend to them. We often need healing, so every one has a god they approach for that end. In Greece, that would normally be Asclepios or Apollo, but on one island it was Poseidon. Any god would be capable of healing their worshipers, though.