Purpose is the point the author is trying to get across to the reader. The author wants something out of the reader when writing and this could be achieved through various methods such persuasion or just being informative. Generally when writing, there is always a purpose within it and you usually want to reinforce it with something. Otherwise without the purpose, there is no reason for anyone to read what you have wrote.

A good example of this would be the lab report I did. In order to get straight into the point I had to state my purpose immediately to get the readers attention quickly. Below is a screenshot of this.

(Figure 1)