Basicly what this is for, is a multipartitioned disk, or dual boot disk. There are program overlays that allow you to make a hard drive jump through hoops, like Speedstor.
As a tech, you need to know that the "fdisk /mbr" command should not be used to restore the master boot record on an overlay or a dual booting system.
Here is a page from MS on it.
Usually you will run accross this issue after the fact. The person runs "fdisk /mbr" on a overlay or dual booting system and wants you to fix it
For most cases, "fdisk /mbr" should get the drive back into shape if the MBR gets messed up--viruses use to mess with the MBR a lot.