ummmmmmmmmmm because they're different shapes and sizes than what you already have?!
oooorrrrrrrrrr because they can be used quicker than a cricut and you can get just one rather than the whole tiny tabs set at once?!
Although, that last reason is lame because you can cut a little piece of paper to get just one cut out of the die, but still, by the time you do the cutting and lining up and sandwiching and rolling, you could've already cut 10 with your new punches.
Thats all I've got.
Enjoy your new punches