You're comparing apples and oranges. LaTeX was designed to do high-quality typesetting for scientific papers. It is not a light-weight substitute for a word processor. It was intended to generate large, complex documents with lots of mathematical formulas, citations, tables, and figures. The idea is that it allows the user to focus on the content of the document and not have to mess around with the appearance too much. In other words, you tell LaTeX what the document should contain and LaTeX takes care of making it look good.
If you're asking yourself why LaTeX is better than a word processor or just writing HTML, then you obviously don't need LaTeX. If you're not writing any large or highly technical documents, then there really isn't much advantage to using it.