This is the characteristic of a close packed layer in a crystal. In ice, these layers are not quite flat, because of the internal angles within the molecules - so the crystal is made up of crinkly layers. The fact that ice crystals often grow as hexagonal prisms reflects this basic structure. Most occur only at very high pressures that cause more compact arrangements of molecules to be the favored lattice. Cubic ice is one phase that can occur at atmospheric pressure, but only at low temperature.
Some scientists speculate that it plays a role in the nucleation of ice from water.
Believe it or not, the origin of ice and snow particles in clouds remains a bit of a mystery. Sure, we know that by cooling water droplets, some of them are going to freeze, due to the presence of dust particles or other ice-forming nuclei. So people have proposed various multiplication mechanisms such as the freeze-shattering of droplets. In this scenario, a shell of ice that forms around a liquid droplet would shatter as the water inside freezes and expands. A variant on this, known as the Hallett-Mossop Mechanism , consists of water drops shattering as they collide with and freeze on hail ice particles.