The difference is to turn a large coal generator off, then back on again generally takes about a week. Which makes them completely useless for providing overnight power when solar isn't available.

Coal can only really be paired with something like hydro where you know well in advance that the hydro power plant is going to run out of water.

These days coal power plants often actually pay for the grid to take power from them. They are fine with making a loss during the day if it means they can make a profit at night when nobody has solar. This significantly impacts the financial viability of solar power and is the main reason there aren't very many large scale solar plants in the world.

Because of coal - you can only make significant profits selling power to the grid at night. And nearly all solar power is primarily intended to be consumed by whoever owns the solar panels.

I'd recommend reading the EU's reasoning for allowing both natural gas and nuclear energy projects to receive "green" funding - https://www.dw.com/en/european-commission-declares-nuclear-and-gas-to-be-green/a-60614990

But basically it burns much, much cleaner than coal, and is easy to fire up, so works great whilst transitioning the baseload to nuclear and renewable power.