There are cynical and practical reasons to use LaTeX.

The cynical: While most conferences in CS advertise both a LaTeX and Word template for submissions, if you submit an article in Word, it seems quite likely it will be desk-rejected for being quackery, possibly without even being read. Is this fair? No. Has the scientific/mathematical community lost out on any great ideas due to this prejudice? Probably not. Think of this as the ultimate, final form of the clear plastic binder from Calvin and Hobbes, except Calvin was right all along and you probably can get away with saying that bats are bugs if you write it in LaTeX. This says more about the process of science than it does LaTeX.

The practical: I can think of three pragmatic reasons.

It seems that nowadays, good science is collaborative. Single author papers are rare. How is it that multi-author papers are written? Writing in a text-based markup language allows co-authors to easily send each other changes and keep a versioned history without having to worry about "track changes" and using file names as tags, and worrying about cross-compatability between ancient version of Office on Mac and Windows and that one person who only has Linux and so opens things in Libre.

In my experience, people do want to get into the weeds with formatting in LaTeX for many reasons, good and bad. For example, how can we shave off two lines of text so we're within the page limit (yes, even though the proceedings will all be published electronically we will get desk rejected for being one page over the page limit where the only thing on the last page are two lines from the bibliography, c'est la vie). That is a bad reason. But good reasons could include improving the look and feel of a document, or controlling where figures are placed, or drawing figures using TikZ. Additionally, having the sum total of the formatting in plain text has advantages. Do you think that undo really un-does everything in Word? Can't you remember some times where you do something, but then un-do and then do "the same thing" again and a different result occurs? LaTeX has many foibles but that is not one of them. Though, of course, sometimes I will wish you luck in understanding exactly why what you wrote in LaTeX works the way it does now. But for example, it is possible to define new mathematical symbols, operators, etc. in LaTeX in a way that is not possible in word processors.

The third pragmatic reason is that everyone else already uses it and some day or another your text will need to be turned into LaTeX for publication. I used to keep a lab notebook in Markdown and sometimes I would feel the desire to copy and paste some text from my lab notebook into a paper (if what I wrote was particularly lucid or well written, or was at least a good place to start from). This almost never ended well. If you have a draft of the paper in Word you might easily spend many days converting it from Word into LaTeX. If it's sufficiently complicated you might never get it converted. This could be a barrier to getting collaborators or submitting at all.

I think that everyone that works with LaTeX, knows, deep down, it is absurd, and the arguments the author runs into are not really arguments anyone would stand by if pressed. I think if anyone is going to write something entirely in prose, that they will never care about the versioned history of or want collaborators on, and don't care if the finished product looks good or bad, you can use a word processor, or at least, this is my criteria for whether or not I use a word processor. Am I drafting an outline of how we're going to write the proposal? That happens in Google docs. Am I actually writing the proposal? That happens in LaTeX, because I want control over the typesetting to fit in the page limit, I want it to look good, and I want collaborators to be able to edit the document and send me plaintext patches rather than docx with track changes maybe or maybe not enabled.