From my understanding, Marine air capabilities are typically for supporting marines on the ground. If a bunch of marines engaged in combat need air support NOW,

From my understanding, Marine air capabilities are typically for supporting marines on the ground. If a bunch of marines engaged in combat need air support NOW, its a lot more efficient for them (meaning, fewer jumps in the chain of command) to have their own planes than for them to have to call up the Navy and ask them to do it.