As far as I know, interception of civilian aircraft is done to assess a situation where the safe status of a flight is uncertain.
A Vueling A320 got intercepted some time ago when the French ATC weren't able to communicate with the pilot. The same airline some time later got another interception by the Dutch. Intercepting the aircraft and watching the situation inside the cockpit and/or the cabin can help to assess whether there is a serious situation developing (and further actions can be taken) or just an ordinary human error/technical fault. I think it's just a matter of gaining some time if a hijacking occurs.