Hey Guys,
I can do a write up for you, it's my view of what I know and how I use it in GT rather than factual and real use outside of GT;
Trail braking is used in all sorts of motorsport around the world more so in motorbike and such, but you'll see it more in high grip cars but the reason they use it is kinda different to how we use it in GT.
First off think of just a right angled corner, and that you never knew trail braking existed. You'd come to the corner, say brake at 100m, stop braking, turn, accelerate. There's a part of that entire combination where all your doing is turning, your not braking, your not accelerating just turning.
Now in a high grip race car, the rear won't really step out unless you over do it, but if you can now brake at say 80m and brake a further 20m into a corner you've lowered the brake marker, your doing something while turning and then you can go straight from braking to accelerating. You essentially gain time from the 100m brake marker to the 80m brake marker, because you maintain more speed from 100m to 80m (20m at any increased speed is time), imagine 10 corners of a track 200m of constant speed increase = alot of time.
So why don't all drivers do it in real life? Well similarly as shown in Gran Turismo the less downforce you have like a GT car your going to have more concerns over grip, a tyre can only do so much work so it's a compromise between braking and turning. But you then have to look at GT car vs say F1 where downforce and centre of gravity is different.
Trail braking effectively is limited by the weight transfer and tyre, if the tyres being told to brake and turn you'll hit a limit and one will compromise the other, depending on how much of a compromise you make depends on then how much time is lost/gained. So some corners may be worth while, while others may not be, a good example would probably be a high speed corner vs a hairpin corner (Rolling is inevitable and you need as much turn as possible).
The weight imagine braking in real life in your own car, everything in the car goes forward, your weight the cars weight any objects all due to momentum. So now imagine this as you turn and brake, all the weight is over the front, nothing is over the rear so the rear is now very light and becomes very loose because of little weight, essentially causing a slide or spin which is one of the main reasons at driving experiences your not allowed to touch the brake during turning because they have no idea how your car control is and it's very risky if overdone. It's also why in real life especially in GT cars from what I've seen racing and any thing I've seen they tend not to do it as your in usually for a long race, tyre wear is effected and any risk is not a potential gain in the huge long run so brake, balance throttle on the turn and then accelerate. F1 the rear end downforce should keep the car stuck and allow you to trail brake in, you will still get a small drift but the downforce should keep the car and tyre stuck (You can see this in slowmo cams but it's not very obvious).
That weight transfer can help FWD cars gain additional grip as a tiny bit more weight over the front allows the front tyres to dig in, although you can quickly overdo this and have to much weight over the front, and then to much work to do.
Now Gran Turismo wise. We have no tyres to worry about, and the car understeers more in this (Generally a game will oversteer/understeer there's not really a game that can be perfect because there's to many variables to take account of). That's my view and obviously people may have differing views. But as the car understeers and as a few have already mentioned early power helps with getting a quicker lap. How do you counter understeer? with a bit of oversteer. How can we oversteer the car without using a decent handbrake? Trail braking, the weight transfer should make the rear loose. Oversteer as well is more controllable than understeer (Well it is in my eyes and far more enjoyable to react to). But if I can make the car in that right angled turn instantly point straight a tiny bit quicker, I should be able to get on the power earlier, and therefore gain time. So...;
Imagine this now from above: 80m Brake (20m ahead already) --> Trail brake to the apex (Just before in some cases) --> Imagine that I get a bit more of an angle on the car (A small drift) --> Say the car was turning at 45 degrees now at 60. --> I am now pointing more towards the straight earlier so can accelerate --> I could now be accelerating say 5m earlier, and have an increased speed all through the next straight which could add to another 10m+. 35m total, one corner x10 + corners thats 350m. Imagine 350m over a 10 lap race, thats 3500m that's 2.1 miles. That's huge, so the little effect of braking into the corner has allowed us to get the huge time gain.
With Gran Turismo sometimes if you brake flat and try to turn you can't get the weight transfer right and it may not happen or happen to extremes. If it doesn't happen or your struggling to initiate it try a slight turn before the braking. You can try this in an experiment by turning and just slamming hard on the brakes the car should spin completely out if you've turned enough. At this point once you can spin off with it, it's all about controlling it with braking input and turn input and then getting the right combination and remembering that combination for a similar outcome lap after lap, it's why say Ti-tech, Spurgy etc. can continue to improve as it becomes a consistent way.
Of course this may not be realistic in some cases but in the end we have to adapt to the environment given in order to achieve the best. If you don't like the 'drifting' motion I will say this, is that a 17.5 is more than possible with a completely flat driving style.
I hope this helps anyway, sorry if it's a bit theoretical but that's my take on it. Any more questions give me a shout.
P.s. I spent my entire lunch break writing that lol!