Bittman
Why prep everything before turning on the heat?
Do everything you can before the heat goes on. The stove sets the pace after that.
Cooking has two phases with completely different clocks. Prep happens on your time: you can cut an onion slowly, take a break, answer a cry from the nursery. Cooking happens on the pan's time: once the heat is on, the garlic doesn't wait for you to find the soy sauce.
Mise en place — 'everything in its place' — moves all the on-your-time work before any of the on-the-pan's-time work. It's the entire difference between a calm cook and a frantic one, and it matters more as heat goes up: for a stir-fry, the prep essentially IS the cooking.
It also catches problems while they're free. Discovering you have no cumin during prep is a shrug. Discovering it while the onions burn is a bad night.