When you are on a fixed rate you are able to take advantage of the wholesale rate without incurring any out of pocket costs. Your rate is guaranteed to stay the same throughout the term of you contract.
You will most likely have an early termination fee if you decide to change suppliers or return to the utility before your contract is over.
The benefit of having a variable rate is you do not have early termination fess and can change suppliers or back to the utility at any time.
Since you are not under a contract, your price will fluctuate with the market and you do not know what you will be paying until after you receive your bill.