Because it's better when it's better.
LaTeX actually a set of high-level commands based on the TeX language, invented by Donal Knuth in 1977. He wanted to write his main book, The Art Of Computer Programming, on a computer, but he couldn't find any word processing program satistfying enough for his expectations. He decided then to create his own, in a way that respects every subtelty and typographic convention due to the old mechanical tools once used to type, and forgotten by a new generation of electonic.
His program was created to optimise reading comfort and respect the aspect and the content of documents.
In contrast to the other word processing programs, LaTeX is guided by a philosophy called What You See Is What You Mean ; indeed a document LaTeX is a source file containing code, using a language vaguely similar to html.
The source code is then compiled by LaTeX, creating the final result. Thanks to this functionment, LaTeX is able to manage on its own the structure of your documents, to automatically create tables of contents, crossreferences, index, etc. Moreover, the user is able to program and define his own commands, allowing him to spare a considerable amount of time when writing long and complex documents.
Best of all : LaTeX is Turing-complete
Thanks to the only font of LaTeX, created by Donald Knuth himself, Computer Modern Roman, your documents won't remain unnoticed. A perfect harmony between text and mathematics made LaTeX the main software regarding scientific publications.
Instead of slaving away looking for double spaces and any other typographic kink tolerated by Word, you will be able to take time to admire a well laid out document written in LaTeX : everything has been thought for an optimal reading comfort.
To compile your source code, you need a compiler and many packages !
Depending on your Operating System, use the following links to install a LaTeX distribution.For Linux, you should install texlive from your package deposit.
Although you could use any text editor, many editors are adapted to LaTeX, as for example TeXmaker.
Now that you installed LaTeX, you can begin to write your first masterpiece. If you want more information (especially about having a layout compatible with the École polytechnique), you may find this on the page « LaTeX at the École polytechnique » (in the button down there), the page about LaTeX on the WikiX, or any tutorial about LaTeX, easy to find on the Internet.