I suppose it is to clear the way for whatever comes next. Bombing everything to smithereens is par for the course. Every time they do this it seems that the Islamists take over the new governments, and impose the most strict Shariaa Law possible and then start ethnically cleanse whatever territory they take over. The Koran tells them exactly what they have to do and how they have to do it. The results speak for themselves: The New Caliphate.