Exceptions and Preconditions

The most common use of preconditions is to demand a property because it would be hard or expensive to check it.
It is an engineering judgment. What is the cost of the check? what is the scope of the method?
For example, binary search methods require that the array be sorted, otherwise they could not deliver $O(\log n)$. If they had to sort, it would be $O(n\cdot\log n)$.
Even when using a precondition, you might want to check it if it can de done speedily and easily.