Because it works when the Sun is down and the wind isn't blowing, and doesn't pollute like coal does. It mean in the dead of winter when there is little/no sun, we can run electric heaters.
Don't let perfect be the enemy of good. We can easily reach +95% renewable grids. The further north the more wind energy in the mix. Which is stronger and more prevalent during the winter.
Lets see what we have left when we have reached 95%, it will not be the issues we think are most critical today.
