Let's say that you are travelling on a US domestic flight and you will not be checking in any baggage.
Aside from gaining access to the security checkpoint, what purpose does check in serve?
What information could the airline gain that it doesn't already know? Since passengers can check in 24 hours ahead of time, they could just as easily not show up.
In other words, why add this extra step? Couldn't passengers without bags just board the aircraft?