Originally Posted by Hitch
However, the reality is: it can't hurt to use it, but the BISCAC codes won't help you with readers. When was the last time you searched for a book using a BISAC? Right? So, if you don't, most won't.
Well, you don't search by BISAC code directly, but at Amazon, for example, when you browse by category, you're browsing by BISAC category. You don't see the code, you see the human-readable discription of the category. Similarly at Sony, B&N, etc. I assume.
So from the reader's point of view, you're absolutely correct. However from the authors POV, s/he'd better supply some well chosen BISAC categories if s/he wants the right eyeballs to find the book. It's important for more than just libraries. I think KDP has you choose the categories from a drop-down list of BISAC descriptions, so you still don't need to know the actual code. Sony, Kobo, and some others, not so much.