When introverts run into a situation that their Dominant Function can't handle, they need to adapt by negotiating, going along with the limitations of low-bandwidth social interaction, or otherwise doing something that does not involve total control or understanding of the situation. The Tertiary Function provides justification for maintaining an Introverted approach directly opposite to the worldly expansiveness that the person is ready for.
Tertiary Fi (IxTJ): "I can't possibly go along with this, because it would mar my soul. It's not 'me'. I am a good person, and in order to maintain my integrity, I need to steer clear of this. This is the responsibility of those other people: it emerges from their souls, not mine, so it's their problem." The Secondary Function (Te) would say: "Do something. Take responsibility even if no one gave it to you, and go after some tangible gain right now, within the limits of the situation and your current understanding. That will improve your position, after which you can re-assess and plot a new course."