When you process your order by clicking the GET Your WordPress Theme link button; you shall be getting a copy of the theme under GNU GPL License, i.e. you will not receive any Purchase Key, Updates, or Support regarding your purchase.
Whereas, when you process your theme order by following the BUY WordPress Theme link button, you will be redirected to the Theme Forest Website where you complete your order through Envato Account and you shall be getting the copy of the theme from the Original Developer and the theme pack consists of Purchase key and applicable support and updates for a specified period through the Theme Forest.
So, if you are willing to have a Licence Key with your theme pack, you should follow the BUY button link during the order processing.