In almost every contact to me, for either agent representation or consultant for self-publishing/marketing development of a non-fiction book, the question shown on the title line is part of the discussion. It is pretty obvious to be able to do enough research on your own to establish data on who or where is the potential market for your book and within that target market your view on who would want to buy/use your book. For example a book about business usually is done about a particular section of it......and your research needs to identify, if you can, the size of the market within that section. If you can't find the data then you'll need to fall back on your credentials to give your opinion as being knowledgeable about that section.
Target market can sometimes be made up of the "core market" ....for example a book essentially to establish setting up a philanthropic activity for medium-sized businesses would have a core market made up of human resource managers of medium-sized businesses. The broader "secondary market" would be considered human resource managers for businesses of any size. "Core market" data is often the most important since its size helps to validate it is large enough to reach out in terms of marketing/publicity in a direct way and project a good enough result for the manuscript to be published.
"Target Market" knowledge is very important and it is worth the effort to be able to know it when developing a manuscript as best you can. Good luck
Many of you contacted me to help provide opinion on credentials to author the book idea you have. I hope it helped.