In the short term, it means you're just keeping fossil fuel power plants online for longer (if you're in the process of transitioning). You stop burning oil in your car though, and it's still a win, though in terms of ROI not an amazing one.

In the long term, once you're generating so much renewable energy that it's causing instability in the grid and you can't pump more in, BEVs are a probably an essential component of handling that.

Certain grids at certain times have already reached this point (the SA battery being a great example of where BEVs could have helped), but most are many years away from it, depending on how the price of solar trends.