You can already check in 12hrs prior for a United flight jumpseat, unless you mean check-in at the gate…. because everyone loves a foot race in uniform to the podium. As far as last minute listings….schedules change, plans change, weather changes. A primary flight cancels or is weight restricted now making a pilot 2 leg it home through an airport they had no intention of ever having to go through. Seniority is what we have all dealt with. That Skywest kid still gets on before me on his metal as far as I can recall. Be friendly be professional. Sometimes things don’t go your way, especially with American who’s gate agents never know how to find your commuting pilot booking.