1) The amount of things you can do in VR is more expansive than any medium before. Video conferences for the most part is used to catch up or transfer information faster (or at least that's how I use it, to catch up with friends/business who are far away). However, it's tougher to use video chats to build NEW experiences, and I can only really think of Google Hangouts and playing something like WarLight/editing a doc that does that. Humans for the most part build better relationships when both parties have shared experiences, and in VR you can actually do a lot of things that you could in real life. This is why it's so different from just "videoconferencing"
2) It finds a balance in anonymity and not having to commit 100% to a conversation. For example, when you video conference you have to pay more attention/be more aware of how you're acting, which explains why many times we choose to have text convos rather than just calling the other person. In social VR you're just an avatar so you don't have to care as much about your appearance/interaction/subtle facial expressions etc. The outward behavioral bar is lower so you can relax and enjoy the environment even more.
VR is a powerful medium because it addresses the above two points - you don't have to be as concerned about your appearance/interaction when you're an avatar AND you can actually do more tangible things in VR to actually BUILD better relationships.