Nesting is useful where you want to apply an effect as a global, where applying the effect to each seperate element would not work well.
For example….
you build a multi-layer result in your edit…
then you want to Page turn out of that multi layer effect.
By nesting, the software sees those multi elements as a “single stream” and the page turn can be applied…. applying the page turn to each element with then effect wouldn’t give the same result.
Yet you can still step into the nest to make content changes, which makes it superior to rendering (pre-baking a file)..
does that help?
Alex