Orbital bombardment lets you technically conquer the world, but it's not worth much.
-drops royal capslock- That was getting old anyway. But yeah, I'm mainly just conquering this semi-populated asteroid for mining the carbon and other minerals. The water would be nice, but there are a few rocks a couple parsecs over that I could get it off. A "fair" ground battle with the dominant species isn't worth what I would get for it. Plus more thinking and planning is made obsolete by their pitiful orbital defense grid. I mean, they don't even have any kill-sats! What do they expect to do if anyone comes knocking? It's just bad hospitality.
Save the bombs and mine somewhere else. Maybe the Moon, if you can't reach the asteroid belt.
Now stop glassing my new territory. I'll give you an alliance. You get some mining rights we could discuss, and we both agree to free trade and to help the other's empire if attacked.