I need help to understand when to use useMemo hook in React vs useState + useEffect. From what I've understood after reading other questions on S.O. and websites :

• With useState you can create a value (changeable through the setter) and you can use useEffect to do whatever you need if some of the dependencies listed in the dependency array changes. On first render for example your component will be rendered with the initial value passed to useState, then the effect will run and your component will re-render with the new value you set with the setter in your effect.
• With useMemo you can create a value that will be computed from a function you give to the hook and it will change if some of the dependencies in the dependency array changes. So far, to me, it looks like useState + useEffect except you don't have the setter to change the value. On render your useMemo will run if one of the dependencies has changed. Difference here : your component will wait for the value to be computed before rendering (not sure about this part).

From what I've read, useMemo should be used for heavy operations. But if it's blocks the rendering of the component what's the point ?
Some says that with the dependency array you can avoid unnecessary updates but I can also do it with useEffect without blocking the render ? If I don't have a heavy component to render, isn't it faster to use useEffect?

As suggested in the comments you can take a look at this question for a global comparison.