Necessary? Possibly (or perhaps even probably) not.

But certainly beneficial. He consolidates his power and eliminates possible future enemies.

That notwithstanding the fact that there is a line in the movie, spoken by Tom to Sonny, I believe at the start of the war (which I'm too lazy to look for), in which Tom makes some reference to the fact that "All the other families will be against you."

So quite possibly they were, and Michael felt that he had to eliminate all of the other family heads.