Why Realm_tgs is needed in the Kerberos protocol (step 2)

Kerberos message exchange

In msg.2, the AS sends Realm_c, ID_c Ticket_tgs, K_c-tgs, times, N1, Real_tgs, ID_tgs encrypted with E_k_c.
I do not understand why Real_tgs is included if AS and TGS are on the same machine.