Basically, the "role" of the functions change a LOT depending on what the dominant function is, because that function is the ego and everything is in relation to it.
The primary difference Jung saw in personality was introversion/extroversion. In order to be extroverted, you have to suppress your introverted side & vice versa. Of course, with age, people begin to differentiate their other functions, but this why the inferior causes problems (it's been suppressed to develop the ego). The auxiliary differentiates & develops as a support to the ego; I would consider it an expansion of the ego as it allows you to extrovert or introvert (depending on your dominant type) without your ego being threatened.
After I/E, the second major distinction Jung makes is rational or irrational, aka, judging or perceiving. So for 4 letter type, it's not just functions, but also whether the person is Pe-dom or Ji-dom (for xNFPs). Even though INFPs have a "Pe" face of sorts, they're dominant rationals & ENFPs are dominant irrationals. *This doesn't mean Ji-dom are more like Je-dom. Remember that Ji has to suppress Je in order to develop.
As much criticism as MBTI may get for the whole J/P thing, that's likely one of the reasons they indicate type with that dichotomy. It still makes Ji-dom "P" types because they prefer Pe, but in testing IxxP, it's meant to indicate Ji more than gauge how much you like to interact with people or organize stuff .