There isn't really one, you can just build stuff and collect resources if you want. Some people consider travelling to the End and killing the Ender dragon as beating the game, though, but lots of people aren't interested in that and just like building. It's basically a sandbox where you can do whatever you like :)
There are lots of mods to make the game different, some people use it for roleplay, Hunger Games, factions, and so forth.
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