It's useful sometimes. For example--you tell Word to generate a Table of
Contents (TOC). You look at the TOC and decide that the Part I, Part II
headings would look better centered, while Chapter 1, Chapter 2, etc are
left-aligned. You center the Part I title, and because "automatically
update" is checked in the appropriate style, it automatically centers
Part II and Part III as well. This is nice.
But a Word-generated TOC uses special styles (TOC1, TOC2, TOC3, etc)
with no other purpose than a TOC. For more multi-functional styles,
"automatically update" doesn't work so well.