Hierarchies
When we consider restriction operators, there is always a predicate involved (either a predicate on data in the case of selections or a predicate on metadata otherwise). The predicate helps us to identify the subset of the cube which we want to pick.
It would be useful to have some predefined subsets of dimension members or cells and explain what we want in their terms.
Of course, we could use member numbers for such predefined subsets, for instance, members with odd numbers, etc. However it is not very intuitive and may not satisfy the purpose which drives our desire to view the world multidimensionally. That’s where hierarchies appear.