Impurity amongst the people of God is a big deal in the face of a holy God.  And as God is about to depart with his people on a journey he wants to know if they are going to be faithful.  The book of Numbers will show us that they aren't and in fact that they can't be.  Here we see the true nature of God, who pursues his people relentlessly and ultimately in Jesus deals once and for all with our impurity.