I'm a minister of religion and I use LaTeX for both sermons and theological papers. I keep all my work in version control and the plain text format just makes so much more sense than a word processor.
I'd love an excuse to put some mathematical notation into a sermon one day, but I'm yet to find one.
As this question is unanswered so far, I will try my best to answer it. But I think, that because the way the question is worded, that no definitive answer is possible, so my answer will contain some portions that reflect my own opinions.
Also - as Joseph Wright pointed out in the comments - you may want to have a look at Why should I use LaTeX? and What professions use TeX/LaTeX besides CS?.
(La)TeX is great for producing texts while concentrating on the content and without too much distraction from caring about design. LaTeX can have a neat design, but the main focus is on producing text. So for essays you should be fine. Be warned, that LaTeX is not totally easy to learn, if you have to get ready an essay for tomorrow, don't try to learn LaTeX now. But if you learn it, you may save time later, because you can make all your essays with the already predefined macros and style.
You also talk about graphic design, and that may be a little more difficult. While LaTeX is able to make nice designed documents, most designers prefer to see direct output. That means, if you change something in your design-program, you see an immediate effect on the document. In LaTeX you always have to compile the document first. So, if you already have a license for a design-program and seem to able to use it, you may better use that for your design-work.
Is it worth learning LaTeX? In my opinion yes, especially if you produce many texts: reports, books, articles. What kind of people doing different subjects/occupations use it, and why? Hard to say, who uses it, but it is clear, that LaTeX seems mostly be used in science. Why? I think, everyone has another answer. For me is it the focus on the content, and doing design other days. I can even separate styling-issues in different files than the content. If I already had a solution for look for one document, I can simply copy/paste it into in similar document for same look. And as LaTeX uses simple text-files, I can use all programs for working with simple texts, from Unix command-line tools to version control.
Best Answer
In 1993, I was drafted in the army (at that time, army service was still mandatory in the Netherlands). I was placed at a communications centre that ran 24/7. Being a communications centre, we had access to quite a lot of good PC hardware for that time. Also, typically nothing much happened, and so we installed the then brand-new Slackware Linux distribution on a bunch of them, to toy with and study Unix system administration (as that was a lot safer than experimenting on the actual srv4 minicomputer we ran).
Slackware at the time came on 40 or so floppy disks, and about a quarter of them were marked 'TeX'. I remember thinking: if it is considered that important, it must be good ...