Thank you very much for playing my game!
Every shell modifies the crab's base attributes. For example, the can shell makes it slower and causes it to fall faster, the rainbow shell makes the crab double all its attributes, and the spiky shell resists more damage while making it slower. So each shell itself gives you different attributes!
And to make the game harder, if you reach the boss with a shell, you have the chance to fight them using the shell you picked up previously. However, if you die, you lose the shell and will have to fight without one (might be a bug, but it's a good feature lol).