There are numerous occasions, especially during late night flights, when hardly there is rush at the airport, the pilots park the airplane out there somewhere, and ferry the passengers by bus. Is it a big trouble to take the plane to the aerobridge and then take the plane to the parking spot?
An airport will have a menu of charges for the services it provides to airlines.
Some of these charges are unavoidable - landing fees, for example - but others will be subject to agreement.
An airline can save money by opting for a cheaper option in certain cases.
If you've ever watched successive plane-loads of Ryanair customers climb out of their 737s into horizontal rain to trudge across 100m-wide puddles to the terminal, and each subsequent shipment of shivering human cargo queue at the steps as the wind whips the standing water up to knee-level, while every other airline during the same period uses a bus to deliver its passengers, you'll have had a very visible indication of they way airlines can choose to pay for particular services - or not.