They have a hinged jaw and compete with other predators for food. Bluegill have a small mouth that is only big enough for bugs and small invertebrates.

They only spawn once as versus bluegills who spawn multiple times per season.

They spawn before bluegill and bass so they are there picking off the fry.

Now for the rest of the story. I think they are a beautiful fish. Much more colorful and pretty than a bluegill.