You work for them. The thing is,you need those requirements to be able to get those chests. While grinding for sellables (example' woodcutting)instead of "You gained 90 woodcutting experience and 1 fossil log",you'll see "You got a wooden chest!!!!!". Each chest depends on the job you're doing. Ashes chest is for firemaking,Minerals chest is for mining,Water chest is for fishing and wooden chest is for woodcutting.