The most important thing is how detailed they are. Man pages contain everything one needs to know for unix programming. If applicable, they specify standards conformance and document behavior that standards leave undefined or implementation-defined. They document system calls, functions, files, programs... Everything a capable user could ever want information on. They provide detailed technical information.
Another nice thing about man pages is how they don't require an internet connection. They're usually installed alongside the packages they document. This is useful if the system is disconnected from the internet.
I don't think the man command itself is essential to unix man pages. I think the detailed documentation they provide are the most important thing. One could install documentation in HTML format and use a browser for viewing -- wouldn't that be just as good?
In fact, git has an option¹ for doing just that. Passing --web instructs git to display the manual page in HTML format in the user's browser of choice. It could be a graphical web browser or a text browser such as Lynx.