Further explanation about why side-effects should be limited to useEffect

[HunderlineK]

There are multiple recommendations about when side-effects are okay and when they are not okay; it is difficult to memorize them all, so it would be helpful if some further explanation is added explaining exactly why useEffect should be exclusively used for side effects e.g.
https://github.com/reactjs/reactjs.org/blob/master/content/docs/hooks-effect.md#example-using-classes-example-using-classes

For traditional life-cycle hooks, the reasoning is that some of them might be called multiple times before the render is committed; is the same applicable to hooks?

Thanks!

p.s. This comment seems relevant:
#302 (comment)