Suppose you know that a certain fairly determinate idea is presented in a certain (fairly determinate!) book, not in general over the course of a long argument, but over the course of, at most, a couple pages—perhaps stated explicitly on just a single page. You don't just at the moment remember which page it's on, and haven't got the book at hand to check, but are eminently confident that it's in there somewhere because you just read it recently. You might be tempted to cite the work in question and claim that the idea is discussed on "p FIXME", reasoning that you'll get back to it later.
If you do that, you could find that when you go back to fix the reference over two years later, you have far, far less of an idea what the correct page is, since in the interim you haven't looked at the book in question even once, and are faced with the prospect of either just removing it or engaging in an unexpected and uncertain amount of reading.