Why a virtual appliance?

Easier to install, use, and maintain. And safer.

A virtual appliance is ready to use. Not a pile of parts. No assembly needed. Tested.

Easy to install. Just boot it. You get all the software you need, the right versions, pre-configured.

Easy to use. No training needed.

Easy to maintain. Automatic updates and professional support.

More secure. (soon) Systems are isolated from each other so an attack on one isn't as likely to harm others.

Virtual machines are resistant to APTs, Advanced Persistent Threats. Some attacks compromise a machine at a very low level, in BIOS, driver PROMS, etc. Even reinstalling the OS doesn't help.

But a properly configured vm can be safely and automatically recreated with all data restored. This is much cheaper and easier than replacing physical machines.