Live meetings typically tend to start later than scheduled due to late arrivals. Tardiness doesn't necessarily undermine physical meetings entirely, but it can make video conferencing software more difficult to use effectively.
As Inc. Magazine notes, participants are at their computers, so there's an inherent risk of them navigating to other applications if they are kept waiting for an online meeting to begin. Some participants may move on entirely and not participate in a conference at all.
Not starting on time, without giving advance notice of the change, is also unprofessional. If you're using a web conferencing application for business purposes, you'll appear disorganized and unprepared to participants. Associates and clients expect punctuality and anything less is a negative reflection on you and your company.
Web conferencing software is supposed to make meetings more efficient and that isn't possible if digital meetings don't begin on time. Video conferences should start at their schedule time. While some participants may log on late, a host shouldn't wait and force those who were on time to do the same.