Hello
You craft because you need to make things...and you need to make things to avoid the monsters...Like swords, and then you need pick-axes to mine...Then you need to craft to make things for traps! To trap all the monsters and the like...You then make the trap so that when you kill the monsters you can farm from them...to make more things! Like bow and arrows and other things! (cant remember what Ash is used for)...
You can then build a portal to another realm (hell essentially) and move through that if you dare
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It's just fun
Then if you go into multiplayer on servers you know (like ocuk) you can help in community projects, or just add to the city being built...or go off and build you're own thing