I've written a piece on why we have a market for frequency ancillary services in the modern electricity market (Australia specifically).
Building up from first principles using an (over)simplified example. I explain what happens to a simple system without control before introducing control systems and how AEMO coordinates our grid.
https://primal.net/e/naddr1qvzqqqr4gupzq9pqdfnf38nagatwdnxxgwv2qphx2plzkftehlhjaeu6qe5fwfw0qqhhw6re94jx7tthv5kksctkv5kkzttdv9exket594nx7u3dveex2ut4v4hxx7fdwdjhyanfvdjhx0cve3rlz