All of the things sister comment listed and more.

* Compartmentalization

* Layered defense

* Windows is not allowed to touch my bare metal

* Having development images for work, play, etc.

* Easy migration

* Configuring multiple separate environments from one control node

* Accessing my virtual machines over the wire using QEMU on my laptops

* Being able to take a system crash without losing other running contexts.

* Maintaining host-level logical volume management and snapshots while guests retain a simplified partioning layout, with separate images for easy migration of `/home` and `/usr/local`

* Advanced firewall configurations

etc.

* Company image

* Snapshots

* Portability

* Isolation

Just a few things i could come up with as valid reasons someone would want to do this.