Using video calls, you not only hear the other user but are also able to see them through your phone. The same happens for the other user on the video call: they are able to see you while you talk to them.
Video calls are often useful when you want the other person to see what you are referring to or if you feel more comfortable looking at the person you are talking to. A video call also allows you to make sure you are really talking to the right person and not an impersonator. Phone calls are only offering you the capacity to hear but can be more practical for different situations.