They are fun to do and can give you some nice rewards. They set a goal for you if you don't have one yourself and will keep the players busy. Also, completionists love Achievements!
Each achievement will reward the player with something. Usually gold, exp or some blocks but sometimes it can give a unique weapon or other rare stuff.
Type /achievements to see a list of all achievements you can complete and if you have already completed it. Also it would be nice if you can show your completed achievements ( as well as your level and stats) to other players.
Look here for examples of achievements:
This is a cool idea :D
Make them also on steam
Hey Eld., make it some 5 achievements has a 25-168 plats reward
Or rather make it a screen with strange images that fits to them like the shop