In a pure markup language, such as SGML, it is reasonably clear that control over the final presentation lies with the receiver of a document and not with the author.
However, the way that LaTeX is often used in practice means that most people (at least when using the standard classes and packages) expect the formatting to be preserved when they send the document to another site.
For example, suppose, as is still the most common use of LaTeX in publishing, you produce a document for `camera-ready-copy' using the class `article' and that you carefully tune the formatting by, for example, adding some explicit line breaks etc, to ensure that it fits the 8 page limit set by the editor a journal or proceedings.
It then gets sent to the editor or a referee who, without anyone knowing, has a non-standard version of the class file `article' and so it then runs to 9 pages. The consequence of this will, at the least, be a lot of wasted time whilst everyone involved works out what has gone wrong; it will probably also lead to everyone blaming each other for something which was in fact caused by a misguided distribution policy.
It should also be noted that, for most people, the version of the class file `article' that gets used is decided by a site maintainer or the compilers of a CD-ROM distribution. To most users, the symbols article in:
\documentclass{article}
are just as much part of LaTeX's syntax as are the symbols
12pt in:
\hspace{12pt}
Thus they should both define a standard formatting rather than
sometimes producing 1 more page or a 5pt larger space.
Users rely on the fact that the command (or menu item) `LaTeX' produces a completely standard LaTeX, including the fact that `article' is the `standard article'. They would not be at all happy if the person who installed and maintains LaTeX for them were allowed to customise `article' every second day so as (in her or his opinion) to improve the layout; or because another user wanted to write a document in a different language or typeset one with different fonts.