Homeowners aren't signing contracts where they agree to use exactly 450MW of power at a constant rate 24/7 for the entire year. The problem with "Free market" utilities is that they are reliant on private sector contracts like this to fund expansion

From a business perspective, if the grid can handle the residential load 99.9% of the year, paying these businesses to cut usage during that other 0.1% of the time is a LOT cheaper than expanding their service to add one more decimal place of uptime that sits idle for the entire year

Cloud platforms like AWS/Google/Azure do something similar, where you can rent unused servers for pennies on the dollar with the expectation they can be reprovisioned by someone else on a seconds notice