There's no shortage of uses for 'used' ev batteries that outlasted the vehicles they were in. They are quite a valuable commodity already. There's all sorts of things you can do with a good quality battery that is still good for a few tens of kwh of storage.

But even when they are still in the car they can be off use to the grid and provide many GW of on demand buffers for both energy shortages and over supply.

There recently have been negative energy rates where electricity producers preferred to not shut down expensive to restart plants and instead ended up incentivizing people to charge their car batteries.

This doesn't even have to be a huge amount of energy per car. Even a single kwh of capacity per day would quickly add up if there's millions of vehicles plugged into the grid.

The same technology can allow the same users to power their house for several days using just a single car and allow them to leverage their own solar panels, get payed to soak up excess grid energy (e.g. at night when wind mills continue producing), deal with grid outages, resell their own solar excess when demand is high, etc.

With million mile batteries, people'll be less concerned about breaking their batteries; especially if they get to spend less on electricity. With leased battery business models, this could be simply part of the deal.