React Class Components offered the possibility to decide whether a component has to rerender or not. It was achieved by using the PureComponent or shouldComponentUpdate to avoid performance bottlenecks in React by preventing rerenders. Let's take the following extended example:

In this case, every time you type something in the input field, the App component updates its state, rerenders, and rerenders the Count component as well. React memo -- which is one of React's top level APIs -- can be used for React Function Components to prevent a rerender when the incoming props of this component haven't changed:

Now, the Count component doesn't update anymore when the user types something into the input field. Only the App component rerenders. This performance optimization shouldn't be used as default though. I would recommend to check it out when you run into issues when the rerendering of components takes too long (e.g. rendering and updating a large list of items in a Table component).