The difference between purpose and function as nouns is that purpose is an aim to be achieved, an objective, a goal, while function is what something does or is used for. Entertaining plays are not always light and silly; there are also plays that are very sad or exciting. I think it will be useful when I am dealing with clients because I need to be polite to them and to help them or get orders for my business I need to know what they need and why (audience and purpose). The purpose of a text indicates the reason why you are writing a particular document.