Main reasons are:
You will learn much more about how things work, examples are networking and suchlike, XP etc. does pretty much everything for you, whereas in Linux you really get under the bonnet.
Easy to use, with a bit of intelligence.
You can go as fast or as slow as you like. I've set up Linux servers and desktops with very little Linux knowledge and found it a dream to use. Even a fully fledged SUSE Linux Server, you can add users, make folders, use Samba to create a network, all in easy steps bit by bit, no annoying Windows dialogue boxes popping up etc.
Rewarding to use.
Comes with 1000's of applications.
Also, you can use Linux without even installing it. Just stick a Live CD in, boot up, it'll go into Linux, give it a go, it'll run in memory:
http://software.opensuse.org/