Try writing a 300 page book in HTML, create citations and bibliographies, table of contents etc. Then convert it to a pdf, say using Pandoc and then you will have a better understanding where TeX/LaTeX fits in. Write also some complicated formulae using MathML. Save the document and render it again in 30 years!
TeX was designed to typeset a document always the same way on any machine that could run it. You can typeset any of Knuth's papers from the 80s and they can still compile today. Maths as Barbara is fond of saying has a long shelf life.