Manpages still matter for Linux systems. Not necessarily for GUI-only tools, but any command-line tool should have a manpage. They remain the most convenient and consistent form of documentation for command-line tools, with the possible exception of --help.
If you need to know which command-line option to use for a particular behavior, consider the relative amount of time needed to find out via a browser and search engine, versus installed documentation, versus a manpage. And if the search engine comes close, half the time it does so by finding the relevant manpage (or excerpting the relevant manpage snippet).
The troff markup language remains terribly baroque, and in truth it hardly matters except as "the thing that existing manpages all use so the man tool handles it". Nothing but inertia and existing infrastructure keeps that format necessary, but learning the small subset of it needed for manpages doesn't take long, especially if you copy an existing manpage as a starting point.
Groff knows how to produce HTML from a manpage, and the result can help if you want to provide documentation that works on other platforms.
The conventions of manpages matter far more than the markup details. If "man sometool" popped up with troff-formatted documentation that didn't have the expected sections and section order, it wouldn't have anywhere near the same value.