The main difference VoLTE provides is preferential network access for voice calls. In times of congestion, voice calls will still work when data might not.

VoLTE calls also have a coverage advantage to them over data. Cellular networks have different handover settings for VoLTE (QCI 1/5) where the device can hold onto its RF longer while a call is ongoing and still allow the call to work using techniques like TTI bundling. The codecs used for VoLTE require very little bandwidth and thus can still operate in extremely poor conditions.

And if you're wondering why carriers don't just offer the same for data, it's because customers using data in cell-edge cases would be disappointed with the experience even with the extra handover settings and TTI bundling. That combined with the extra PHY resources being allocated to a situation where data just might not arrive 70% of the time makes it more worth it to allocate those resources to VoLTE specifically especially when they might be critical like in E911 scenarios.