Linux is growing every day also for desktop usage, but it must be the first choice particularly if you use computer for:
Programming
many programmers use Linux to develop applications, particularly about C, C++ and Java programming languages but also scripting languages like Perl and Python.
Web development
Linux is the optimal operating system for web sites and web apps development, many CMS recommand the use of this O.S.
Hacking
Most hacking tools are designed for Linux, there are also specific distros for hacking like Backtrack.
File server
Linux is a very reliable operating system with a robust file system, it’s suitable to be used as file server.
Media server
for video-audio streaming using DLNA applications like PS3 Media Server and Twonky Server.