That is why we have whats called "spinning reserve". It does not necessarily mean you have reserve generators spinning all the time, it can also refer to any backup or reserve power source that can be bought on line within ten minutes notice. This is how they compensate for generator down time whether planned or unplanned. It also is used to help absorb very heavy industrial loads, some of which are tens or hundreds of megawatts. Steel mills and Aluminum plants use enormous amounts of energy, some well over a hundred million watts (arc furnaces can themselves consume over 100 megawatts.) I have even heard of an electrical motor used to drive the fan of an extremely large wind tunnel consume 44 megawatts. And you just cant turn them on, you have to call the utility and tell them to are so the can switch on the spinning reserve if necessary. This ensures there is enough power for the plant and for the grid.
And as for why they leave generators spinning is simple. It is a very time consuming process to stop a high pressure steam turbine to bring it off line. To stop one you have to gradually slow it down and then gradually cool it down. Then you have to purge the water from it that condensed otherwise it will destroy the turbine blades on start up. Then to start it you need to slowly heat it up and then speed it up while making sure no water is present. So to make life easy and keep the reserve ready at a moments notice they lust let them run with no load. They are only stopped for repair and maintenance.
I am not saying you cant have spinning reserve with renewable energy.It just not possible to do it with the most popular forms such as wind and solar. No wind or sun light = no spinning reserve. You could use batteries but how economical is that and do we want buildings full of toxic lead, acids or other hazardous metals? That many batteries will need allot of maintenance. And there will need to be a rotation system as battery banks wear out with time so you must be sure your battery banks can be ready to handle a load. Other methods of reserve are stored water reservoirs but how easy are they to build and what about towns or cities that are on and surrounded by relatively flat land?
Most of the power generated in the world comes from high output plants that use a fuel source to boil water. Its going to be a while before we find something that can be able to handle: high base loads, provide "spinning reserve" and that fit into relatively small spaces and are economical to build and maintain. Until then wind and solar are great supplements and will help curb some emissions, but not eliminate them.