When you defeat Meta Man, all you get is the power to have the game yell at you after each level about everything you could have done to finish the level more efficiently.
Hey, that could be a helpful skill!
"Man, i'm stuck, how do I get past this level? I know! I'll use the Meta Buster!"
*equips Meta Buster, fires*
"Dude, use the jump dash here and get the helmet upgrade then go use Floral Cannon on Space Platypus, you moron!"
"Ohhhh, now I get it. Thanks, Meta Buster!"