according to the inspector, why does the devil send deviations to us

From the text:

The Devil sends Deviations among us to weaken us and tempt us away from Purity. Sometimes he is clever enough to make a nearly-perfect imitation, so we have always to be on the look-out for the mistake he has made, however small, and when we see one it must be reported at once. You'll remember that in future, won't you?'