The reason why this is done is because major industrial customers (e.g. data centers) often require upgrades to utility-owned infrastructure when they set up shop. Utilities will typically require what they call a 3rd party capital contribution (i.e. upfront cash payment) for some portion of the total cost, with the remainder to be recovered via their margin on the additional electricity consumed over the lifetime of the facility. You would also have the option of paying the full cost and making no commitment as to whether you'd actually use it.

If a customer moves in and says "oh yeah, I'm gonna need an average of 100 MW 24/7 for the next 20 years" and then turns out to only use half that, the utility is now short a bunch of a money that it spent upgrading its distribution infrastructure for that customer's supposed needs.