That's it, isn't it? Man pages are great if you a) already know the name of the program you want (c'mon, apropos/man -k have never really worked that well), and b) have at least a vague idea of how to use it.  If you want to look up what a flag does, or whether there's a flag that does what you need, they can't be beat.

But they aren't great for either exploration or discoverability.

Now, this is less of a problem when a command takes its input on stdin, writes its output to stdout, any errors to stderr, and it has, like, five or fewer different options.  For the use case of programs-which-are-really-filter-stages designed to be connected via pipes, and when there aren't all that many of them, it works fine.  Once something else is on the table, yeah, the man page is at best incomplete and at worst basically useless.

Say what you will about VMS, its HELP functionality makes it much easier to go from "I want to do X" to "And here's a sequence of commands that will do X" when you don't already have a good mental model of what's on the system.

Adam