What's the point of checking in online for a flight?

Think of flying like sending cargo. You are the cargo. By checking in online, you show an intent to get on the flight, if you get a boarding pass at the same time. At some point you will have to show your ID and also get a boarding pass. If you check in at the airport and not pay for seat selection you get what you get. Do not complain. When you fly no frills, you get what you pay for. Not much but a flight to your destination.

When you check in at the airport, then you are included on the manifest, the list of passengers. When you show your boarding pass when loading on the plane that confirms your name and presence for the final manifest or passenger list for the airline.

Good luck

The most important reason for checking in online is so you don't get bumped if the flight is oversold. Most airlines generally bump the last passenger(s) to check in.

Most airlines also strategically block seats until the day of departure. One advantage of checking in online and checking the seat maps is that you can see when the seats open up.

Most airlines also offer a mobile app which means you can download your boarding pass to your phone. And yes, whether you do that or print the ticket at home, you can just go straight to security unless you're checking a bag. you still have to stop in at the kiosk if checking.