Reason behind trasit Vlan

[knaik99]

why do we use transit Vlan on L3 LAN core  interface  and Vedge i.e. WAN as we can give IP directly on interface also?

or

why do we use transit vlan between two LAN core interface  which are directly connected to each other  i.e. intercore link and we give IP via Transit Vlan

Please explain

[balaji.bandi]

There is a good explanation by @Joseph W. Doherty 

https://community.cisco.com/t5/switching/transit-vlans/td-p/1803574

[knaik99]

Please explain about exact use of transit Vlan and its exact purpose as I can give IP directly on physical interface.