Imagine if, when people plug their big battery into the grid, the grid and the millions of big batteries talk to each other to negotiate electicity flows. Then the EV fleet could help even smooth the power supply and shift load and supply to match. Sun comes out, loads of EVs are charged at that point. A bit of cloud. The EVs give some back. To incentivise you could choose a cheaper per kwh "keep me topped up if you can" and more expensive "definitely fill the battery asap" and even a "hey not using this car for a bit, make use of my battery and pay me!".

This is in addition to, if the battery is plugged in at home with solar panels, the home can self-smooth and do a lot of this locally, giving the grid the net in/out it desires from the house based on sum of: battery, appliances, solar as a whole unit.