As Whatever pointed, developping my tertiary Fe was, at first contact, a necessity. I just needed to be less immature and unsufferable, so I could get more easily accepted by most people. And furthermore, it also allowed me to spend less energy coping with stress and societal norms (I think norms and bureaucracy are never really understood by ENTPs, and hence they often represent a very big issue for us).
So, I'd say this function first developped as a tool, from a very utilitarian point of view.
But then, I'd say that in my late teens, some (tragic) personal events forced my tertiary to evolve more quickly than expected. This was a true revolution for me, when you suddenly become conscious of everything you missed so far.
And by now, my Fe is supposed to be excellent (almost on a par with my Ti), according to the cognitive tests and my close friends (who are more concerned than mere tests, aren't they?).
And here is the curious part: because as I said earlier, my Fe was first rather utilitarian, but now, well... Now I'd say I'm beginning to feel a real empathy towards people, even if I can't understand or justify why. It's not faked or forced anymore. It just comes more naturally than in my memories.
And what's even stranger, is the fact that the more I use this Fe, the more confortable and confident I get, the more I feel "like a responsible adult" (although a very weird one, nonetheless ).
For me, my tertiary Fe manifests itself like a spontaneous drive to help people wherever I sense that this help might be needed. I'm more aware of what's going on around me, and can now react within a split second. I'm more patient and more tolerant than ever (1), and feel more at ease when I have to display my emotions in public. I've learned not to repress them any more, or almost.
Again, it doesn't mean I've accepted societal norms, since I'm still very awkward with them: however high my Fe could be, I do not think I will ever use it like a natural Dom or secondary user.
Because when I use my Fe, it's almost in a childish way, in a very pure form, thus I think I may paradoxally look more sincere than they are (2).
And while speaking of them, I'd also say that my communication skill with children greatly improved. 15 years ago, let's say it wasn't so easy to be around them.
I'll conclude by saying that as a university lecturer who cares a great deal about the students he's responsible for (especially during their thesis), my tertiary has eventually become an essential function to keep my audience captivated, motivated or simply entertained!
(1) Well, not always when the subject is purely academic.
(2) Perhaps that could explain why the theory says that the older we get, the more visible our tertiary is when we superficially deal with people?
In the same way, my father, who is a Dom-Fe user (ENFJ), looks more and more like a very well-grounded, stable, rational intellectual [Se-Ti], even if I know that his true nature is rather very seductive and quite temperamental. He was one of the leaders of the maoist movement during the may 68 events in France; a rather wild youth, isn't it!
"A man who only drinks water has a secret to hide from his fellow-men" -Baudelaire
Those of you who have developed these functions, how do you believe their growth has affected your perception/interaction of the world?
You can take them very far indeed... and it's not just limited to tertiary and inferior... you can equally work on the shadows.
I've also noticed in terms of development... you can get a better (or I should say, a more complete) grasp of your tertiary function than your inferior. For me, my use of Se is "trained" but only in certain aspects.
There's two main aspect to Se awareness: detection and discrimination. I'm quite good at detecting and locating objects if I put my mind to it. But discrimination (e.g. telling something is different... focusing on all the details) I do very poorly on.