Villagers are human like mobs you trade with. Emeralds are used as a currency. Most fletchers buy sticks from you. The more you do business with a villager, the more trade options they have. High level fisherman buy a boat (5 wooden planks) for one emeralds.
Tool smiths start selling high quality enchanted iron and diamond tools once they are mid level. Enchantments do things like increase durability (unbreaking), improve speed (efficiency), and many other upgrades. Once you have a tool smith that sells say diamond axes, anything else is pointless to keep beyond an emergency backup. Cost isn’t a factor because the aforementioned selling of sticks and boats mean emeralds are a cheap and renewable resource.