As Kineto Wireless points out, for a mobile operator, the assumed technology for providing voice over LTE is IMS. The problem with IMS-based VoLTE is that:
(1) The specifications have not been completed. There is no agreed approach.
(2) IMS is an all IP technology, and there is no circuit switched connection. Mobile operators on the other hand have circuit switched networks. As such if they go for LTE they would be forced to invest in new/expensive IMS telephony equipment.
With VoLGA, mobile operators will be able to re-use their existing MSCs to do VoLTE. LTE transport network supports 8 different priority queues, so mobile operators have the ability to prioritize voice (or other services) at different rates. VoLGA will be just another packet application running over LTE.
VoLGA does not dedicate bandwidth to voice. That is part of the LTE radio network. The primary advantages of VoLGA are the ability to reuse the existing voice network for voice services over LTE, the ability to bring voice to LTE from the start.