If you're passing the callback as a prop to a child component, then it will cause that component to be re-rendered every time the parent component renders unless you memoize the callback. This happens because React checks referential identity when determining whether props have changed. The performance hit of unnecessary re-renders can be fairly significant in large React apps.
React will rerender the child component anyway in this case even with useCallback. Of course you can prevent this by using React.Memo on the child component, and in this case using useCallback makes it possible to optimize rerendering by providing stable values for functions you pass to that component.
Adding useCallback for functions only does something useful if you also React.Memo those children. So while this is a very important optimization, it's not one you need to apply to everything.
Memoizing the callback prop only matters if you wrap the child component in "React.memo".
If you don't wrap the child component with "React.memo", every time the parent renders the child will render regardless of prop equality, even with memoized props/callbacks.
