It's hard to describe the purpose of the game; you make of it what you want. You can hop in multiplayer and socialize with others, or play solo and try to survive hordes of mobs with style. In the end, you do what you want, and try to find enjoyment out of it.

I just find it amazing that the freedom of minecraft allows players to create their own ALUs (granted, on a giant scale).
http://kotaku.com/#!5650027 - This guy went and made a functioning 16-bit ALU. Machines run by machines run by machines.