I personally just don't find much use for the hammers in regular play, however giant hammer is a great trump card to have. If you're 100+% and they're really low %, pulling out the hammer can net you a free kill without much risk (relatively speaking) because you're already at kill%. They can't hit you with any attacks or they die, because of the infinite super armor, so the only thing to be scared of is a kill throw, or a throw that sets up a kill. Otherwise, them landing a throw is just another 10ish% on top of your already kill%. No biggie. Them baiting out a swing that misses might get you killed, but really, the risk/reward is very much in Kirby's favor. If they mess up their bait or whiff a grab, they die. Nothing survives a giant hammer. Think of it like going for a risky gimp when they're off stage at really low % because you dying at 100+% wouldn't be a huge loss. Kirby also weirdly has complete control of the match when he's got a giant hammer out. The opponent cannot do anything they would normally do until that hammer goes away.
There isn't really any other move like it. Nothing except Mac's KO punch has the opponent scared of losing a stock at 30% from one mistake. And no other move let's you take complete control of the game like it does. "Alright, stop everything, we're playing this game now. Don't **** it up."