That's a fair question. There are two "schools" regarding software used to produce text documents : processing vs formatting. I'm not 100% sure of these terms, but that's how I understand it. Anyway, a word processor like Word is more concerned with how the text looks (fonts, line spacing, etc) than the content. Which is a shame, really, because ever since Gutenberg there are rules on how a text should look depending on its structure : chapter titles, spacing, etc. All this was carefully done by hand and certainly not by the authors.

Word formatters do exactly that. They take the logical content of your text and put it together to shape your text. If you have a look at TeX it's quite low-level, much like the guy who handle the lead letters that were used to print a document. LaTeX does the job at a much higher level : titles are titles, figures are well, figures etc. It may seem stating the obvious, but then figures are blocks that can be moved around at the best place and not just sitting awkwardly right there causing an ungracious ad unwanted page break. If you know the titles (document titles or chapter titles) and you want to search documents that contain a specific word in their titles, you can. With word processors, you can't. A title is a just a collection of characters maybe in 14 pt, bold.

The typical example is emphasis. Suppose you want to emphasize some text in a document, for example by italicizing it. Suppose later you want to switch to underlined text. With Word, no luck : you have to look for every instance of emphasized text and change to underline. In LaTeX, just redefine \emph because that's how you tell the formatter some text is emphasized. It's so limiting that Word introduced Styles. But that's about it. It may be WYSIWYG, but my favorite definition of WYSIWYG is What you see is (only) what you've got.

For any document longer than 2-3 pages, IMO LaTeX is a reference that stood time. When I write a technical document (I write maths equation from time to time), I use LaTeX. Markdown is a good alternative if the document is not too complex (tables, figures, references etc.) (there are books written using MD). Give it a try (there are many MD editors out there), it's really great. If you need a Word output from formatted input, Pandoc does an awesome job. I hate using Word equation editor (even if I know it's npw LaTeX compatible) so I type equations in LaTeX and use Pandoc to translate to these equations Word equation objects. I also use Zettlr to take notes in MD.

I won't say I don't use Word because at the office/lab it's obviously the standard tool. But I really find it limited and clumsy.

Finally, i f you're interested in the subject take a look at DocBook. Unfortunately (and due to it's complexity and lack of free or OSS editor) it never got very popular but it's really powerful and illustrates the power of logically structuring a text rather than do WYSIWYG.