Some of your posts are confusing to me. Which carrier allows you to check in for your flight months in advance? I used to work for an airline and I am not aware we allowed passengers to check in even days in advance. As far as I am aware, you can only check in for your flight within 24 hours of the departure time once the flight has been opened; certainly not months, weeks or even days in advance of departure date.
Easyjet allow you to check in 28 days in advance. Ryanair similar if you have paid seating and so forth. Low cost seem to allow further in advance than the “traditional” carriers buy many of these airlines do allow 48 hour check in like SQ and EK
Early check-in definitely benefit the airlines. As most of the airlines have dynamic pricing and overbooked, from early check-in they can do some better revenue planning.
