Apollo’s diversity is unique among the Greek gods. Aside from music, hunting, and prophecy, he is also one of the sun gods (Helios is the other, and actually is the sun). Apollo is attributed as both a causer and healer of diseases, thus giving him domain over medicine. Since he possessed the gift of prophecy, that lent him to becoming the patron of colonists, who sought his oracular insights to guide them in their colonies.
The other Greek gods and goddesses all specialize, sometimes hyper-specialize, in one or two main areas, but Apollo has bucked this trend. His wide-ranging attributes make him indispensable to the Greek pantheon, and he’s unlikely to be replaced. In fact, Apollo did the replacing, taking the attributes of other gods for his own, and of all the Greek gods, Apollo is the only one to make it into the Roman pantheon name unchanged.
In one respect, his diverse portfolio embodies an aspect of moderation, which he also comes to adopt after performing a year of community service for killing the Cyclopes—the ones who forged Zeus’s lightning bolts, not the ones who tormented Odysseus. It’s said that he learned his lesson, and changed his tune so that he was the spirit of moderation, preaching “nothing to excess.”
This is a fine sentiment; however, the flipside of his diverse portfolio is that he wants it all. He truly has an excessive number of divine attributes compared to the other gods, rivaling even Zeus. So, while outwardly he states his favor towards moderation, his nature from past deeds defines him clearly. More likely, he’s simply biding his time until he can make a move on Zeus.