If GDAX is better why use Coinbase?

Not better, just different. On Coinbase you can buy in one step and lock in a price before your bank transfer clears. You can't do that on GDAX. It's a multi step process where you have to deposit money first before you can think about buying. You also don't have to worry about slippage or different order types on Coinbase. It's all abstracted away for you and very simple.

GDAX is a trading platform, so it might seem a little more daunting (UI-wise) for someone new to crypto.