I assume then the followup question would be why they'd be using a timer while drawing. If it's at a convention I could then assume they might do small commissions for people and use the timer to ensure they draw them quickly and don't spend too much time on each drawing.
Most manga were releasing on a weekly basis. That’s a lot of work with short deadlines. He probably calculated the maximum amount of time per page and times himself in order to be able to finish on time.