The jets get airborne and intercept and airliner for several reasons:
Primarily - to show a presence and ensure flight safety to the other airspace users. Should the Airliner go low level and fade from radar, turn off the Mode C (Accurate height reading) the the jets can report the height and we can ensure everyone else in the airspace is safe.
Secondly - to minimise casualties and, if required - engage the airliner. The idea is, an airplane crashing in a field doesn’t harm the same amount of people that would be caught underneath it in a city.
Yesterday’s event, a hoax - would most probably have been intercepted with the second reason in mind. It’s also an air policing role.
When an airliner loses radio Comms, the jets are airborne, intercept and communicate with the pilot via hand signals while multiple calls on guard are made to the airliner. Compliance turns and an eventual escort are the results of such an event.
The little dink turn and then a second back to the original heading are what we call a compliance turn.