Ditto DRSTEWART's and Dave's comments.
Standby...pretty much a thing of the past, I think.
I did it many times in the late '60s while in school, home and back. Half price.
Cheaper than Greyhound bus. Usually American or Delta, or Braniff, even TWA.
There were so many airlines flying back then, many open seats.
Called the airline (no computers back then), reserved a "standby" ticket.
Usually successful in getting on the plane.
These days when you hear an overhead call for "standby passenger Joe Blow", the guy has missed his flight and trying for another one, so he is 'standing by' in case a seat opens up (a no show passenger, etc.).
Or somebody trying to get on an earlier flight than what they have.
Southwest doesn't play with most of the discounters.