VoLTE vs. Cellular Voice

In today's cellular networks, is there any advantage to having your calls go over VoLTE rather than the same call going over the standard cellular voice network (whether it's CDMA or GSM)?

Is there any advantage to having a device that supports VoLTE versus a device that does not?

Which US wireless carriers today already have active widespread deployment of VoLTE service and how widely available is it?