-It delivers all voice and data services over same network. Hence it does not require to maintain legacy infrastructure.
-VoLTE make calls much faster compare to 2G/3G voice calls.
-It offers better security and QoS (Quality of Service) compare to legacy 2G/3G networks.
-It extends life of battery used in smart phones. This is due to use of shorter DRx which results into improvement in device power efficiency.
-It increases functionalities of VoIP protocols.
-It offers improvement in capacity using RoHC/SPS protocols.
-It provides call continuity between packet switched LTE network and circuit switched WCDMA network using SVRCC (Single Radio Voice Call Continuity).
-It improves UL coverage using TTI bundling.
-All the profiles are GSMA defined and hence it is compatible across the world. This leads to easy roaming, availability of handsets etc.
Fonte:
http://www.rfwireless-world.com/Terminology/Advantages-and-Disadvantages-of-VoLTE.html