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.
LaTeX2e is the latest stable release of LaTeX. When people say LaTeX they usually mean LaTeX2e, not an earlier version.
LaTeX3 is a massive refactoring of the LaTeX2e code base, which is currently in development. Some of the developers are frequent contributors here and will probably chime in. You can follow the development at LaTeX3 GitHub repository.
The difference to end users between LaTeX2e and LaTeX3 will be many more functionalities included "out of the box" rather than requiring packages.
The programmer's interface to LaTeX3 for class and package writers will include lots of enhancements like built-in data structures and methods to control expansion without so many \expandafters. Some current packages use the expl3 package already to implement their features.
Best Answer
AFAIK there is no official certificate. You may get one from a lecturer or a school when you participated in a course.
It would be nice if there was something comparable to the Linux certificate, with certain levels such as:
L1: is able to use the most important document classes and has a good understanding of packages
L2: Is hardcore LaTeX kernel developer.
Well, there should probably be a few more levels.