OP, it's an industry standard practice called Standby; many airlines have policies/procedures for it, usually only offered for free to pax with certain status, maybe to others for a fee.

I assume there are several reasons why it's not simply offered to anyone/everyone/anytime. All that same day changing of flights could put a burden on the airline staff/systems. There may also be some gaming of the system that would be disruptive; business travelers book flights as late as possible in the day and then try to fly standby earlier in the day if their business is completed earlier. I'm sure there are other reasons.