Psychometric instruments don't have purposes.

I don't think this statement communicated well (I almost reflexively downvoted) because "instruments" is a term that typically implies design and hence purpose.

It might be better to say that the purpose of psychometric instruments is simply to reveal truth, and truth doesn't have a specific purpose (it's just useful for lots of things you might want to accomplish).

Compare the statement "an inch has no purpose" to the statement "a ruler has no purpose" -- the latter has the purpose of measuring the former, whereas length itself is just a property of an object in the universe.