Why does bitcoin core need to download the blockchain I understand that the blockchain is what keeps track of who has what coins and makes sure that everything is correct and secure, but why do I have to download it?  Isn't that where the miners come in?  Don't they keep track of the blockchain?
Certainly miners, and many other users, keep track of the block chain.

But if you don't download and verify the block chain yourself, what will you do when you want to know whether a transaction is valid?  You could ask a miner, but what if they decide to lie to you?  You would have to trust them to tell you the truth.

Part of the Bitcoin security model is that you don't have to trust anybody.  When you verify the block chain for yourself, nobody can trick you into believing that an invalid transaction is valid, or vice versa.