One of the purposes, I believe, is to encourage the reader to think about stereotypes and their function in society. We are asked to look at ways in which we might be adhering to negative stereotypes or building our lives around seemingly positive stereotypes only to find out that things are not what they appear to be. This is certainly true of Changez who initially falls prey to the promise of magical meritocracy only to come to realise that there is no magic at all. Hamid challenges the reader to think deeply about aspects of stereotyping and to form our own individual conclusions.
I certainly agree with the interpretation given by 'fuchsiadreams'. I can only add that it may be pertinent to look closely at Hamid's choice of narrative voice. It is a monologue given by Changez and as such forces the reader to view only one perspective - his (which is the stereotyping issues as previously mentioned) However, I believe that implicit in that choice of narrative voice is the authorial voice saying 'Look what happens when there is no dialogue'(in both senses of the word) And what does happen is that misinterpretation is the only possibility - that's why the reader can never 'know' what happens in the end, we can only guess. I think that Hamid is warning us that unless we engage in dialogue we will find the truth to be like the shadows on the wall of Plato's cave. The authorial voice urges the reader to be inclusive rather than exclusive.