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In the CCIE training videos, the initial config consists of a subinterface with a vlan tagged to it.
I dont understand why a "normal" interface with just an ip address cannot be used instead.
Which companies training videos are you referring to?
If you are referring to INE then there is a very good reason for this. It is because they use virtual routers (Cisco CSR1000v) and these are all linked to a single vSwitch on one physical port.
By using subinterfaces you can quickly create new interfaces between routers, e.g. VLAN 12 is only on subinterfaces on router 1 and 2. VLAN 34 is for Router 3 and 4, and so on.
This way, they only have to setup the physical topology (i.e. connect a single interface to a vSwitch) once - and then you can quickly change the topology on the fly.
Imagine having to recable things if you wanted to do it physically?