You make it sound so mysterious. Speaking for myself, I do it for long conversations because it's actually painful to keep the phone held to my ear for that long.
First of all, just to keep your hand raised up for an extended period of time is tiring. Even without any weight. (Modern phones are actually lighter than the Dialog handle was. )
Secondly, for many people, it is not convenient to touch the ear for a long time with any object.
Also you can send audio recordings in a text message. I think I once read on hn that in some crazy asian country that is the main method of communication: they don't text, they don't call, they send each other voice mails.
cell < landline < skype
For me, having a conversation on a cell takes some effort, and it's tiring. A conversation via landline is much more comfortable, and skype is best. This is all due to the voice quality.I like Skype or Hangouts. I think the latency is a lot lower. When I'm on the cell, I feel like I'm always talking over people.
Also, voice transmission only happens when the phone detects a voice, and this has the effect of clipping off the very first part of your sentence. I'd find this highly irritating, but I've noticed it a lot less in the last few years. Perhaps that has been fixed.
I agree that mobile voice quality is terrible on most phones, especially when people choose to make their calls on speakerphone for some reason. At work we have landlines and actual desk phones with hard-wired headsets because the call quality is amazing compared to mobile. It lets you have a fully nuanced, tonal, relaxed conversation with everything but the facial expressions and hand gestures.