It also means there aren't any servers in-between hosting the call, and that no one is mining your data and information, or is even aware that it's happening.

Over SMS your carrier would be aware of the action, and could stop or censor that interaction if required by an outside force, not to mention that there is basically zero protection of your data with SMS.

I may be wrong, but I think the idea here is the censorship resistance, privacy from prying data miners/advertising, ease of grouping a bunch of these services together, and no reliance other outside parties to get any of it done.