Fighter jets escorting planes after various sorts of emergencies seems to be standard procedure in many countries, you hear about it quite frequently. It's sometimes implied in the media that if the situation would turn into a 9/11-type hijacking it might be necessary to shoot the plane down but nobody seems willing to fully clarify who could take such a decision and under what conditions on the record.
Now, in most cases, there isn't even a suggestion that a hijacking is happening but it's difficult to determine that in a timely manner and there are still two things fighters can do in other cases:
- Get a visual confirmation of the situation aboard the plane (Is the plane damaged? The cockpit windows obscured? Who is in the cockpit?)
- “Guide” a pilot who has lost communication to an airport.
Also, one factor is that being able to scramble jets is often seen as a basic requirement to assert sovereignty (witness the mini-controversy in Switzerland when it was revealed that the air force could not do it at certain times of the day) so politically it seems difficult to entirely give up on it, even if it could be argued that it's a waste of money.