Some airlines explicitly require it and even charge "penalty fees" for non-compliance. If it doesn't work or isn't available - for example on charter flights - many passengers get upset. We're talking about online check-in, which is now almost indispensable. In the past, it was generally common practice and in most cases there was no other option: passengers had to go to the so-called check-in counter with their ticket, which was more like a small booklet, and their ID. There they were given their boarding pass and checked in their luggage. Business travellers in particular, who even in the "good old days" often only travelled with hand luggage, often found it annoying to have to stand in queues and sometimes lose valuable time. Some carriers have reacted to this and introduced so-called telephone check-in. The data then had to be given, the boarding pass was then issued and initially deposited at a collection counter at the airport. Nowadays, only a few carriers offer telephone check-in and if so, only for frequent flyers with the highest status and/or first class customers. The boarding pass is sent by email where possible or handed out by an employee at the airport. The exact details and instructions for telephone check-in can vary depending on the airline and airport. Passengers are usually provided with a special telephone number that they can call to complete the check-in process. Regardless: Telephone check-in was the precursor to today's internet check-in. Alaska Airlines was the pioneer The world's first airline to allow its passengers to check in online