I mainly use it for starting up secure shell connections (ssh) to other unix machines. I used the command line non-stop on my linux machine, but have fairly well adapted to the mac's habit of having a GUI for everything, and using the GUI is now faster or about the same for most operations I do.
I don't tend to use the default 'terminal' app, though. The replacement 'iTerm' is way nicer, IMO, as it lets you have tabs, transparent backgrounds, etc, etc.
iTerm
To answer the OP's question, though, there are lots of more server-oriented programs, like mail spoolers, webservers, etc, that are often more easily managed via the command line than via any kind of graphical interface. Or that don't have a graphical interface at all. The way most unix tools were written, is to write a bomb-proof program that has only a command line interface, and then people come along after that and write some kind of graphical front-end that interacts with that command line interface for the user. Often the graphical interfaces don't have all of the functionality that the command line interface does, so many power users just skip the graphical stuff completely.
Or, maybe this guy just doesn't like the Finder...