When I am the student, I prefer to have sessions with webcam because we have body languages that help a lot as Gerald mentioned, especially in Portuguese, without understanding our finger movements, our facial expression, can be a bit tricky. An example given, in many languages to say that something is excessive, they say " too much " in English " demasiado "in Spanish, " zu viel " in German, but in Portuguese we tend to say the standard word " muito " but to give it the sense that it's " too much " we make a facial expression to show the negative side. 

Not to mention that the articulation of the mouth and some pronunciation exercises are much better when there's a webcam.

And also, a very important thing, sometimes it's not good to translate stuff, it's much better to show the action if possible or to show an object so video is perfect for that. 

Being a student, I request cam, if teacher does not have it, I probably won't book a session. 

As a teacher, if my students don't like or don't have webcam, I only turn it on in order to show something that it's better described if showing them with a real object or action as I mentioned above. 

I talked with two people on Skype. The first one prefered talking face-to-face (with webcam I mean), but as it was my first time, I felt a bit embarrassed because without any reason, my face turned red, so I wondered how my face would look like. Thank God that we couldn't look into each others' eyes (this is not a romantic situation), since we tend to look at the screen. Then I forgot about me, so we had a nice conversation. :)

Another person just called me, without a video call, so I accepted. You just get used to talking like that, you don't mind the way you look like or the position when you are sitting. Everything is natural, but at the same time, if that person can't pronounce a word properly, there's nothing you can do, because he/she doesn't see your mouth. That's the drawback of listening without watching. Many facial expressions are missed when something funny occurs, or you would like to enjoy the conversation by being with someone else that's not from your family.

I definitely prefer talking, if I have the chance, 'face-to-face'. :)

I teach (English, French and Hebrew) using the webcam, but if the quality of the sound is not good I turn off the camera, and then I ask the student if he/she wants to turn it off too. I do find it is helpful to see the student's face and reactions and for him/her to see me, but if the sound is not good we can manage without it.

For me, I find it a little offputting to speak to someone and not see their face. I don't like talking on the phone for that reason, even in English. Facial expressions and gestures are very important in communication, and I would always prefer to see someone during an exchange.

I can often tell if I have made an error in pronunciation or grammar by the other person's reaction when I speak. This helps me to correct myself and removes the necesity for them to do it, which I think is better because the other person doesn't have to keep breaking up the conversation as often to correct.

And I think that works both ways. I will give a very simple example. This morning I had an exchange with someone, where they said something along the lines of 'They have a lot of childs...' I guess my face must have screwed up a little when she said it because she stopped for a second, thought, and then came out with the correct plural, which is 'children.'

Without that, I would have had to have interjected and I don't think that is particularly conducive to a good exchange if you are continually having to step in to correct. It's also good because you don't have to 'spoon-feed' the right answer to the person, often they can come to the right conclusion themselves if they just have a second or two to think.

So for me, having the webcam on just makes communication easier, and I'm all for that.

So far, when I had a session, my teachers never ask me to use the webcam. I never asked why, even I would prefer to use web cam because I did not have any idea about what he was doing while I were speaking. It made me wondering if he did the other things while teaching me.