In costume people are often brasher and more outspoken than they would normally be. The costume hides their real identity and they find their inhibitions reduced. Suler has written an interesting chapter specifically on male gender switching in cyberspace. Men seem to pretend to be women in order to inject sexuality into interactions. Suler has some intriguing questions to help detect male gender switchers.
Women sometimes pretend to be men in order to avoid the possible injection of online sexuality.
I assume that it was to avoid adding any element of flirting or sexuality to a professional relationship. The Internet is the great equalizer. A high school student can correspond with a Nobel Prize winner on an equal footing on the net. Behavior Online is a good example of this principle. Mental health professionals and others can interact online with some of the most prominent clinicians and researchers in the field.