Link is an oracle, which is a middleware that is necessary for smartcontracts. An oracle is necessary to bring that from outside the blockchain to smartcontracts, so smartcontracts can do something else than ICO and cryptokitties.
For example, do you want to make smartcontracts to buy stuff online, and when the product is delivered it will automatically transfer the payment to the sender. You will need an oracle for that.
Chainlink is one of the few decentrralized oracles. It is a nice idea. Nobody knows if it will be successful or not. I think that buying now, at the current price, is risky. Too much speculation already, and it is nearly at its ATH