How did this conflict encourage the expansion of Macedonia under King Philip?

Answer 1

After the Peloponnesian War, a defeated Athens became a second level power. The Greek cities continued their usual pattern of fighting each other under various coalitions, with at first Sparta dominant, then Thebes which defeated it. Persia intervened, imposing the King's Peace to stop the fighting spilling over into its territories.
This fighting allowed the rise of Macedonia in the background. Philip II was able to secure his borders and consolidate his power in northern Greece and then turn his attention to unifying Greece for the campaign against Persia.

As the eminent J. B. Bury Writes:
"As the hegemony or first place among Greek states had passed successively from Athens to Sparta, and to Thebes, so now it passed to Macedon. The statement that Greek liberty perished on the plain of Chaeronea is as true or as false as that it perished on the field of Leuctra or the strand of the Goat's River. Whenever a Greek state became supreme, that supremacy entailed the depression of some states and the dependency or subjection of others. Athens was reduced to a secondary place by Macedon, and Thebes fared still worse; but we must not forget what Sparta, in the day of her triumph, did to Athens, or the more evil things which Thebes proposed".