First, the GNU operating system needs a desktop environment that is 100% free software. GNOME satisfies this need.

The second goal, by way of the first goal, is that people who use free operating systems will have a nice user environment in which to work (This works also on other non-free UNIX-based systems).

Finally, GNOME has a lot of really neat features that geeks are guaranteed to enjoy. Says one reviewer of this FAQ:

Todd, GNOME has technical merits even beyond "open source" and "nice environment" and "world domination," as you well know...For instance, the COM clone and the use of CORBA for inter-operation gives me a woody.

This is certainly true (the fact that GNOME has great technical merit; I have no knowledge on the woody front.) Try it out for yourself and see!