On the list of phrases that strikes fear into the hearts of students everywhere, "group work" is only slightly behind the clear winner "public speaking." However, group work is often a component of college courses, including online courses.
The reason why professors require group work is the same as why they require public speaking--employers want their employees to have those skills, as well as other related skills developed by working in groups/teams (and public speaking).
According to the 2019 Job Outlook report by the National Association of Colleges and Employers (NACE), the top 8 (ranked) skills employers want:
All student, whether online or face-to-face, need opportunities to learn and develop those skills.
Working successfully in teams generally requires both communication skills and problem-solving skills, and students working in groups usually have opportunities to display initiative and develop leadership skills as well.