Woosey is right.
Think of a calibration file as not one that calibrates the frequency but the SPL measured at a given frequency.
You need this ability if you want to target a curve of any sort. Otherwise, you won't be guaranteed any accuracy.
Let's say your mic has a natural rise in response above 5khz (most do). If you don't have a calibration file to compensate for this and flatten this out, then this rise will show up in every measurement. In a fictional world, let's say your Car measured flat. If your mic has a rise above 5khz, you'd measure an apparent rise above 5khz and you'd start cutting it to flatten it (or whatever you'd do to target X curve). You'd be cutting something that's caused by the mic; not by the car.