The CRO is initially used for staking if you wish to get the ruby or anything above (2% + ). If you do not want to buy CRO you can opt in for their blue card(dont remember the name) for 1% returns.

You load it with fiat (british pounds in my case) and spend British pounds. Rewards are being paid in CRO and you can either just keep them, convert to your preferred crypto or sell for fiat!