When an intimate relationship, sex can be a spiritual, mental and emotional expression (hard to explain but I hope you get the drift). It's where two people can really be intimate and enjoying at the same time.
Jacking Off = You + Your Hand
Sex = You + Her Hand j/k You + Her
See the difference? It's one of the ways to maintain a healthy relationship although not really mandatory in some cases. Sex in a "real" loving relationship surpasses physical meaning.