Purpose is a concept that some writers don't understand. The idea is that readers hope to gain something useful from your document in that they're ;looking for a “walk-away” message; our job as writers is to provide it. Most of us have read documents where we get to the end and think, “Why am I reading this?” or “What is it that I’m supposed to do with this document?” I call this the "and" factor, as in “And I’m supposed to do what/think what about this document?” In technical and scientific writing, this is crucial, as readers don't want to spend a lot of time trying to figure out what they're supposed to do with the document.
To ensure that our readers don't have "and" moments, as writers we need to ensure that we have a concrete goal in terms of what we want our reader to do with the document and what it is we're trying to achieve. To put it simply, we need to consider the action that we want our readers to take and our “walk-away” message.