If we look carefully we will realize that they are two different concepts. Broadly speaking we could say that the "purpose" looks ahead and generally has to do with ourselves, with our personal intentions or objectives, while the "reason" usually looks back and often has to do with other people's impositions upon us. Some people mix them and when you ask "why?" they answer... "in order to..." or when you ask "what for?" they come up with "because..."
The Beatles have an appropriate song to illustrate this question: "Tell me why" (lyrics)
And of course they also have a song to illustrate the answer: "Because" (lyrics)
