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?
The four stages define what the basis of enlightenment is, namely, losing the belief in an inherent self ("stream-entry") ; and, at the same time, further define what full enlightenment is ("arahant"), namely, the complete uprooting of greed, hatred &amp; delusion. Therefore, if you or a teacher (guru) still has lust, hatred or selfishness then this means full enlightenment has not been reached. 

For example, there are many intellectuals (e.g. Sam Harris) who speak and act to give an impression they are enlightened because they speak theories about "no self". However, if we examine their other views and beliefs, we can work out if they are really enlightened or not. 

Or there is a certain chat site where members claim to practise "jhana" yet they also post about their sexual lusts. Since a master of jhana will be at least a once-returner but often a non-returner, any significant lust should not arise in a once-returner or non-returner. 

The same applies to us. The four stages are a guide to us of the level of (our) enlightenment. They are not for boasting but for the opposite. They are so we do not overestimate our experience.