Writing and drawing a regular Web Comic is much more work than most people would suppose; even on a once-a-week schedule it can be difficult to keep up, and five to seven days a week is enough to drive an author away from the series entirely. Part of the difficulty comes from the immediacy of the medium: a webcomic can be, and often is, drawn the same day it gets loaded to the site, and a slip in timing can easily result in a strip getting delayed... sometimes for weeks.
To prevent this, assiduous webcomic artists will try to draw several days or even weeks worth of strips ahead of time, posting them according to the schedule, while continuing to draw the later ones apace. As any computer science student that has already studied the producer-customer problem should know, this provides a buffer against delays, allowing the artist to catch up on late work without the posting schedule being disrupted.
Maintaining a buffer means that it's very difficult to be topical, since it means that months could have passed after the event being referenced. It's also a lot of work, and some authors find the wait between drawing the comic and the readers' responses to be unbearable. However, such work will usually pay off, as most fans are much happier when the series posts regularly. It also allows the artist to take vacations or attend conventions by using the buffer rather than forcing the artist to take time off from their trip to update the site. All in all, a buffer is generally agreed to save more trouble than it causes. The syndicates that run Newspaper Comics have typically made them mandatory for that reason.
See also Schedule Slip, which is likely to occur if no buffer is maintained.