useCallback is quite similar and indeed it's implemented with the same mechanisms as useMemo. Our goal is that ExpensiveComputationComponent only re-renders whenever it absolutely must. Typically whenever React detects a change higher-up in an app, it re-renders everything underneath it. This normally isn't a big deal because React is quite fast at normal things. However you can run into performance issues sometimes where some components are bad to re-render without reason.
In this case, we're using a new feature of React called React.memo. This is similar to PureComponent where a component will do a simple check on its props to see if they've changed and if not it will not re-render this component (or its children, which can bite you.) React.memo provides this functionality for function components. Given that, we need to make sure that the function itself given to ExpensiveComputationComponent is the same function every time. We can use useCallback to make sure that React is handing the same fibonacci to ExpensiveComputationComponent every time so it passes its React.memo check every single time. Now it's only if count changes will it actually re-render (as evidenced by the time.)
Try removing the useCallback call and see if you get the the count to 40+ that the page crawls as it updates every second.
