A USP is the most important thing a copywriter, in fact anyone including the owner, needs to know about their business.
Fail to find this and you will be unable to answer the question if someone asks you "Why Should I Do Business With You Rather Than Somebody Else" (ie your competitors).
Fortunately it is easy when you know what to ask for.
Of course you will be well versed as to why if you have purchased the "Ad Camp DVD Home Study Course" and listened to "Advertising Breakthrough" mp3.
http://copywritingmagic.info/downloads/introduction.zip