These questions are the beginning of writing. In finding the answers to them, you've found something worth saying. Every story, e-mail, policy statement, brief, letter to a client, blog entry, memo to staff, report and note to a friend has an audience and to keep that audience, tell them something worthwhile. Don't waste their time.
Who cares? Consider whether what you're writing is going to the right person. Consider why that person should read what you have to say. Find the words to make your reader care. If you can't, maybe you don't have enough to say at this particular time to this particular reader.
What does it mean? This is a two-prong question. First, your message needs to have some substance. Secondly, it needs to be written carefully and clearly so it can be understood easily and fully.
So What? The answer, or lack of answer, to this question sets the course of when and what you write. Don't waste someone's time if what you have to say doesn't really matter.
Yours in waste management, WMW