No installation hassles, no worries about losing data during an upgrade. No getting stuck in the middle of an install and not having a machine to use to track down documentation.
Just a running machine. Easily, quickly, and best of all temporarily.
There are numerous Live CD versions out there. X-Terminal emulators for running systems without hard drives. Educational software. Game packages. Audio-editing suites. In fact, I've seen so many that I cannot keep them straight.
Let's not forget the power of having a fully functional Linux system in your pocket. Even if you are a happy Windows user (I've heard of them) the ability to just pop in a known clean OS and then recover your data, scan for viruses, or any other operation you may need without booting Windows, is priceless.
Priceless and gratis. They can go together.
Years before Knoppix came out, I thought that server system security could be enhanced if all the binaries could be run from CD rather than the hard drive. That way even if the system was badly hacked, simply rebooting would put the server back into a known good state, running a known clean OS and applications. The worst that could happen would be the loss of some data, rather than having to question every binary on the server for trojans, viruses and back-doors, or erase everything to try to keep the infection from spreading. It's actually easy to author your own Live CD, with only what YOU want on it, for your desktop or your server. This is just about as hardened a computer system as it is possible to create without building a Faraday Cage a-la Gene Hackman in Enemy of the State.
In summary, just search for the Desktop Environment you want to try (Xfce, KDE), or the distribution you want to sample (Debian, Bodhi), or your particular interest (gaming, audio processing, security, system rescue), and add "LiveCD" to get a list of projects and distributions with which to scratch your itch.
It really is that easy, and it's fun. When's the last time your computer OS was fun?