Students can send papers any time of the day, from anywhere. Some students learn better in an anonymous setting, without the distractions of the physical presence of a tutor. Tutors have more time to think over their response. Students can print the tutor's remarks and study them as often as needed. Students get experience working with web forms, email, and learn to read comments about their writing in writing.
Tutors have a lot of guess work to do.
Some errors might be difficult to handle in an online session.