Thread title pretty much says it. Obviously the airline needs to know who, and how many, of the people who are booked on a particular flight actually show up for it - not just for security reasons but also for things as mundane as overbooking and seeing if you need to put any passengers on other flights. But today’s check-in doesn’t ensure that - with all these online check-in and mobile check-in and whatnot options, which you can do a day or so before departure, it’s perfectly possible that someone checks in for a flight and yet doesn’t show up. I can also see that you have to do something with your luggage if you want to check it, but that doesn’t explain why people who travel with hand luggage only have to undergo a checking-in process.
To me, checking in is just an annoying extra process that you have to perform before you fly; I agree it’s not overly annoying, but on the other hand I don’t see the added value that it creates, so it could just as well be skipped: When you book a flight, you are given (either physically or electronically, with a barcode to print out) a ticket which simultaneously serves as your boarding pass. So what’s the purpose of having your passengers go through a procedure which doesn’t seem to amount to anything more than “Yes, I am booked on that flight, as you know, and I really want to take that plane?”
My personal theory is that it’s just a leftover from the time when you’d check in by personally reporting to a desk, so the procedure ensured that you were actually physically at the airport prior to departure. But is there an added value of it in today’s world as well?