1. with an OS inside a file, you decouple the os from binding a partition. This way, you can easily transfer to another pc, etc.. For example, you can pre-install and pre-config some apps, then give the vhd to your coworkers and friends.
2. That depends on the OS, and how you configure it. It boils down the hardware drivers, if you pre-install many drivers, you can transfer to different PCs to boot. Safe to say, for Windows OSes, you can transfer to PCs with similar hardware devices.
3. Virtual DVD is not for real CDROM device, it's mainly used for .iso or other files with different DVD format. This way, you can mount a file without using a real CD/DVD drive. These days, most data cd comes with a .iso file, you copy this .iso file to your netbook which does not have cdrom drive, and you will be still able to browse/read the file, as if it's from your real cd drive.