The first eukaryote cell was the result of two prokaryotic cells merging together, such that one cell ended up inside another, which is nesting. Later these inner cells specialized into organelles such as Mitochondria.
Nesting is a fundamental principle. Nested structures, concrete or abstract, is the basis of everything.
Numbers could be thought of as nested abstractions.
The only way to establish an order of evaluation in a lazy language is via nesting of function calls, which is an implementation of the causality principle itself, if you think about it for a while. No wonder Monads and other things desugars into nested function calls.