1) If you buy OEM licenses, you cant reuse them in the event of a hardware switch because the license is tied to the original mobo.
2) You must have a windows OS (XP, Vista, Windows 7) preinstalled to use VLK. Once you have a VLK copy of Windows 7 you're able to install this on as many already legally licenced Windows 7 machines as you want with the VLK key. You can also create a master image, then deploy it with the VLK key. The machine still uses the original Windows 7 licence, but having the VLK agreement enables you to use your VLK key and custom image for easy management and to keep a standard image install. In addition I believe there are benefits associated with the MS software assurance program.
3) If you retire the PC, you can reuse the VLK provided the original Windows 7 license was a transferrable type i.e. retail.