You can't just reward designs, comments etc., you can also Gift Premium perks to other users, such as plus. Therefore, you need to spend 500 credits tho. By the way: You can earn way more coins by completing some more tasks. You find them under your profile in tje section "your coins"

You can use coins by gifting awards to players, car designs, clips, etc.

However on some, in the bottom there is a button that says award you can use thst