Why cannot the Intermediate Structure be again XP, instead of X'?

Question

_{Source: p 167. Syntax, A Generative Introduction (3 ed, 2012) by Andrew Carnie.}

We need these “intermediate” N' (pronounced “en-bar”) categories to explain the items that are conjoined in these sentences.

In Chapter 6, the book does not appear to explain why these Intermediate Categories cannot be XP again. Why are X' necessary? Why not simply reuse XP?
Cannot each X' be replaced by XP?

Answer 1

The three projections of X'-bar theory namely X, X', and XP exist to establish the Head-Complement, Specifer-Head, and Adjunct relations. If intermediate projections were also XPs, then there would be no way to distinguish adjuncts from (multiple) specifiers.