Primary role of IMS in LTE is to provide service based charging for true LTE call over IP network. We call such type of call as VoLTE call (Voice over LTE). Here VoLTE call will not use 3G/2G achitecture to make voice call, rather it will use IMS backbone nodes (PCSCF, ICSCF, SCSCF etc)
Service based charging is facilitated by IMS architecture. Till date operators are providing internet service based on volume and bandwidth (like 1000Rs u get 1Mbps for 50GB data etc). Using IMS backbone for LTE the service providers (operators) will be able to charge the customer based on service they are using.
Like for Skype which is a real time video call there will be different charging and for email service there will be different charging. IMS will be evaluating the service based on the QCI (QoS Class Identifier associated with it.
Hence IMS backbone is a must for VoLTE call and for service based charging.
