useMemo and useCallback are performance optimizations. Use them only when you already have a performance problem instead of pre-emptively. It adds unnecessary complexity otherwise.

useMemo memoizes expensive function calls so they only are re-evaluated when needed. I put in the [fibonacci sequence][fibonacci] in its recursive style to simulate this. All you need to know is that once you're calling fibonacci with 30+ it gets quite computationally expensive and not something you want to do unnecessarily as it will cause pauses and jank. It will now only call fibonacci if count changes and will just the previous, memoized answer if it hasn't changed.

If we didn't have the useMemo call, everytime the ball moved it'd unnecessarily recalculate the answer of fibonacci but because we did use useMemo it will only calculate it when count has changed.

Feel try to remove useMemo and see what happens. It'll cause the ball animation to be pretty janky.