Enabling a token on Radar is using something in the ERC-20 standard known as token allowance. 

Enabling tokens is a requirement of the ERC20 standard. This ensures tokens can't be traded from your address by a third party without your permission.

This is what enables the 0x contracts to withdraw tokens from your wallet in order to send them to the other user in a trade. You can read more about token allowances here https://tokenallowance.io/

While it seems inconvenient it is a security redundancy that adds to the safety of using Radar and other relayers & dex's.