The point is that it gives them control. Control to impose tariffs you don't want - including, possibly, ones that change hour by hour according to demand - and the ability to cut you off remotely if they so desire.
You absolutely do not have to have one. My supplier emailed me to say I was on their list to fit one, and I emailed back to say I don't want one and to remove me from the list. They replied and said I'd been taken off it. Don't let them bully you!