What would escorting an airliner which is the subject of a security threat by fighter jets achieve?
There are multiple reasons:


Yes, they'll shoot the aircraft down if deemed necessary. It is a grim prospect, but sometimes grim events require grim responses - there is little doubt that many lives would have been saved had the 9/11 aircraft been shot down prior to them hitting the WTC's.
It's a threat and a show of force. Not all hijackers / criminals / terrorists are keen on the idea of losing their life. For those, the threat of being shot down can contain the situation and give some control. If nothing else, being shot down would prevent their political message from getting out - you can't really negotiate the release of prisoners (Or whatever) if you're dead on a blown up plane.
The fighters can get up close and personal with the aircraft. This allows them to see it, see inside and potentially communicate with people on board. During an event where the situation is unclear they may be able to report whether the aircraft is under control and/or command of the pilots.
There are internationally recognised intercept signals that a fighter jet can perform - using lights and motion (Rocking wings etc). This will allow them to pass instructions to an aircraft with no radio capability.