This applies solely to performances of sorts or concerts where artists who are not that popular will perform. The tickets may not cost that much but since not all the tickets/seats will sell anyway, it's advisable to wait until the concert/performance is about to begin before buying a ticket because then you could demand for a discount. It works most of the time.
Yes, I remember seeing a non-name band in college. Me and friend waited until after the show started and got in for free. There were a lot of empty seats, so the concert promoter was just trying to fill the audience.
