It seems as if they put the invincibility there just to put it there. As for gameplay and exploration, let's take a look at Kirby's Dreamland 3:
Kirby's Dreamland 3 features partners, many of which you can get by going into an optional room. This changes your gameplay experience and encourages you to utilize the advantages that those partners possess. These partners can use abilities that can help you unravel secrets, complete levels more quickly, or collect Heart-Stars. Even Gooey has an advantage; he can eat enemies and get their powers underwater. The combinations of partners and copy abilities is vast, giving you a lot to experiment with. This, in my opinion, greatly enhances the gameplay.
Kirby 64, on the other hand, features very little of this. Sure, you can combine powers together, but that even becomes a task, seeing as you have to get rid of your previous power first to do so. I've also noticed that many of the combinations just didn't help me much at all. Even if there were one-off rooms, it would still change the pace of the game a bit, seeing as you have the potential to get more powers, health, etc. Additionally, there are no alternate paths, even for getting the Crystal Shards. Each level is stricly from point A to point B. No hidden doors, no real secrets, no change of gameplay (except for a few levels), and most importantly no exploration. I'm a guy that likes linearity in games, but not to the point where there's zero exploration involved. If linearity apparently isn't a problem in this game, how come I see people complaining about linearity in Super Mario Galaxy?
I'm not here to bash Kirby 64. I enjoyed the game a bit, but not as much as I hoped to. I just think that most of the other titles in the series are much better.