This is just my hypothesis, but I think it's for "brand". They didn't destroyed only books but basically everything, because of this many cities surrendered wit

This is just my hypothesis, but I think it's for "brand". They didn't destroyed only books but basically everything, because of this many cities surrendered without even trying to fight back. In that case your civilization was preserved. That's why the mongols were able to conquer so much so quickly, fear build on these horrible actions.