Online checkin sounds like a simple proposition. From the user’s perspective it’s just a button to click confirming they want to check in and aren’t carrying anything they shouldn’t be. But for an airline there are an incredible number of steps that need to happen in the background before that passenger can be checked in. Typically a passenger will be screened by a relevant immigration authority. That authority may then tell us the passenger must have an onward ticket before travelling. We can look at their booking and try and find the onward ticket. Their bag tags for the right number of bags must be issued and their seat on the plane allocated correctly. For groups bags and seats are pooled. For partner airline travel all this must be integrated with another airlines system.
One of the problems with online checkin is the huge amount of information required to be input by the user. This inevitably leads to errors with passport data. And if it’s wrong the passenger might have trouble on arrival. So the online checkin system has safeguards to ensure that the passport travelled with matches what has been typed in.