They are probably one of the most annoying things ever to evolve, and how the species has lasted this long with a defence mechanism like that, we'll never know. It is assumed that Herobrine created them that way, when he assembled his Legion of Death.
The Creepy Creeper, a Prowler from Daaaaaadum, possibly took inspiration from these creatures when he divised his name. Oddly, Prowlers take a form that looks a lot like Creepers when they enter Nowhere.
Like all the inhabitants of Nowhere Creepers have a rather blocky appearance.