Even pleasant and informal conversations can reveal things about an applicant.
For instance, you might reveal how cogent you are in discussing your research interests, or how likely it seems that you will attend a school if accepted. The voice medium also better allows for other, more subjective, judgements concerning one's apparent attitude, degree of passion, and general character.
I have no idea how important any of these things are, but I suspect that professors are looking for students who are able to present themselves well. Talking informally about your research interests is one way for them to get a sense of that ability.