You don't have to go for the standing charge option, but if you don't you will have to pay a higher rate for the first X units and a lower rate for the remaining units.
It's supposed to cover things like the cost of installing,maintaining and reading meters; the cost of billing; the cost of maintaining the pipes/cables. I think there must also be an element to cover some of the supplier's environmental obligations, subsidising social tariffs and covering arrears for those who don't pay.
I can see that there are fixed costs but i think it would make billing and comparisons more transparent, and encourage more careful use of energy, if there was no fixed charges allowed and only one unit rate