Working with styles is relatively complicated (not hard, but distinct form pure wysiwyg picking font&size and hoc) and for most small documents doesn't have much of a benefit in the result.
For larger documents or professional work it's relevant, but by then people are used to adhoc ways
Styles are also b0rken in interesting ways. Have you ever tried to change the default style to justified, and headings to left-aligned? How does that work out for you in tables? Table styles in general lack all kinds of support. Auto-numbering in headings can theoretically be customized, but good luck trying to change it without following a step-by-step guide that ends up with the default numbering. Using styles is supposed to negate the need for empty lines, but the space-before and space-after properties are buggy, meaning you end up just putting an enter anyway. Empty lines also end up in headings since word often, but not always, understands that the 'enter' pressed after a heading should start a new 'normal' paragraph, rather than a mult-line heading. The keep-together, keep-with-next properties on tables/rows work only kind-of, rather than consistently. Good luck autonumbering tables, illustrations and such. Not to mention that if you work on a word document that has had its styles changed a few times, there's a lot of invisible vestigial stuff in there that will interfere with your new styles.
Yup. Yup yup yup. Years ago (so I'm open to the idea that it's got better in the last decade or so - but if we're talking about Office, I will doubt your assertion) I used styles religiously, because I was a younger man who took pride in doing things The Right Way. Eventually, however, the number of times styles screwed themselves (or so it seemed) up eventually exceeded my patience, or my time available, to debug them. Now, I do it ad hoc, and haven't had page three look significantly different than page twelve in years.
If you want me to do it The Right Way, make sure that's a happy path, or I'm going to stop doing it.
Working with styles is relatively complicated (not hard, but distinct form pure wysiwyg picking font&size and hoc) and for most small documents doesn't have much of a benefit in the result.
For larger documents or professional work it's relevant, but by then people are used to adhoc ways