When VDOMs are not enabled and you put your FortiGate unit into Transparent mode, all the interfaces on your unit become broadcast interfaces. The problem with this is that there are no interfaces free to do anything else.

With multiple VDOMs you can have one of them configured in Transparent mode, and the rest in NAT/Route mode. In this configuration, you have an available transparent mode FortiGate unit you can drop into your network for troubleshooting, and you also have the standard NAT for networking.