It’s a commonplace among lawyers that when examining a witness, you should never ask a question if you don’t know what the answer will be. Apparently, there’s an analogous commonplace among doctors—that you should never order a test if you don’t know what you’d do with the results. Interestingly, these two maxims provide superficially contradictory advice: lawyers only ask questions whose answers they know, and doctors only ask questions whose answers they don’t know. But on a deeper level, they get at something similar: asking questions is a purposeful activity, and if you’re not thinking hard about the answers you might get, maybe you shouldn’t be asking the question in the first place.
Doctors also ask questions whose answers they know (or are pretty sure they know), in case lawyers come after them wanting better evidence than clinical judgment based on history and physical exam. Even leaving aside the lawyers, “diagnostic” tests are meant to be used to confirm your best guess — and/or to rule out your worst (reasonable) fear.