Flying in Europe, it is about a 50/50 chance as to whether you get on the plane from a jet bridge or you take a bus to the plane parked on the tarmac. In contrast, most US airports have jet bridges, even when the plane is small unless it is a very small airport.
Why?
Yep, it’s got a rubbery grippy surface to follow the plane. As the plane goes down, the wheel will creep up a little bit so the jet bridge height will go down a tiny bit too.
Fun fact: they also have an alarm if something funky happens, like if someone pulls on the unit when it’s in use.