There are 2 main reasons to stay on top of your energy contract:
The vast majority of energy contracts rollover onto market based rates after expiring if you do not sign a new contract or opt out. Rollover rates are used as an assurance for energy providers. Without a contract you get charged a premium to cover your usage.
Recognizing advantageous times to renew or change energy suppliers. Things like severe weather, regulation changes, seasonality, and the price of fuel affect energy prices.
You never want to enter a new contract that interferes with an existing contract as you will incur cancellation fees. You can, however, sign a contract with a future start date. Let’s say you were in a 3 year contract and 2 years into it the market dips, you can take advantage of that dip and lock in the current cheaper rates for when your contract ends in a year.