What was Buddha's purpose in constructing The Four Stages of Enlightenment?

I can see the major to entering this path as being quite significant: the seeing through ones own imagination of their self. I also see that this four stage progression must be held onto quite loosely as one could become lost in defining their *selves* as sotapanna for instance and living out the identity of being someone who has seen through their identity. However, what practical use can one obtain from using the four stages?