What is an external system with which useEffect allows syncronisation?
These systems aren’t controlled by React, so they are called external. An Effect lets you keep your component synchronized with some external system. such as:
A timer managed with setInterval() and clearInterval().
An event subscription using window.addEventListener() and window.removeEventListener().
Connection to network
some browser API, or
a third-party library, A third-party animation library with an API like animation.start() and animation.reset().