It's primarily a revenue source except under the current ticket rules changing flights would cost more than the $50 Move-Up fee.
I think the normal charges for changing flights is what - $100 or something like that plus any fare difference. That is partially a revenue source and partially a means of keeping people from buying a lower fare off-peak ticket and changing to a higher fare peak flight. The Move-Up fee is a compromise between getting no extra revenue from people changing flights or making it too expensive for people to change flights.