for concerts, re-releases. the performers block seats for their use, and sound/speakers. A few days out, they know what they aren't going to use, and put them up for sale. Requires constant checking of venue websites/ticketbastard, but 95% of the time I can pull good seats to sold-out shows a few days in advance, or sometimes day of show. That's for shows that are not hard sell-outs (sells out day of on-sale). If a show does not hard sell out, even if it sells out, only a fool would pay over face value (i.e., UCONN football and basketball last 5 years, Villanova game at Hartford this year prime example)
At casinos, they will invariably hold tickets for high rollers, and re-release them when there are no takers. Second row center for Stevie Wonder a day before a Mohegan Sun a few years ago. $120. Similar seats were >$600 on StubHub at the same time ;-)