In some applications the actual function and use may be confidential. We manufacture parts for military applications. I am not even supplied a dimensioned print (I have asked). They supply us with a dxf, and we make the part. I do not know what it is or how it is used. I am given a width and a tolerance, thats all. (I had to fight to get that). We are responsible to make the part within the tolerance, and thats all we know. There have been times (before I was given the width) when we had returns. We are not told why the parts are returned, just that they are 'not usable'.
We meet our customers expectation as best we know how, and although I would like to know the function and design of the part, thats not possible.
So there are times when you dont know this stuff.