When to use a Class Component? When to use a Function Component?

Before React 16.8, class components were recommended because they had lifecycle methods (special functions that let you run code during the different phases of a component's life).
But after React 16.8, with the introduction of hooks, functional components can now do the same things — so they became the new best practice.

Before React 16.8, if you wanted to:

Run some code when a component mounted

React to state or prop changes

Clean things up when a component unmounted

=> You needed to use class components because they had lifecycle methods like:

componentDidMount

componentDidUpdate

componentWillUnmount

React 16.8 introduced Hooks — especially:

useState() for state

useEffect() for lifecycle behavior

This made it possible to do everything class components could do, but in a simpler, cleaner way inside functional components.

Most companies now use functional components with hooks.
But class components still exist in older codebases, so it's smart to be familiar with both.