Some bills, like your phone contract, might be for the same amount each month no matter what. But bills like gas and electricity often change, and they might not come at regular intervals.
That’s not a problem. With Direct Debits, you give permission for the company or person to take however much is owed out of your account. They do this whenever they’ve billed you. So the amount and the date can change, and a Direct Debit will still work.