Well, i used to play automatic because it was way easier cuz i didnt have to worry about shifts. About a year or so ago, i decided to manual on time trials only cuz it seems pretty cool. Well, soon it became natural, and now i've always used manual.
Reasons for manualling:
-control the revs during downshift/cornering
sometimes when you want the car to downshift, the fking automatic sticks to say 3rd, when you need 2nd to blast out of the corner. now i found if you lift off the gas and back on it again, itll shift down. But think about it, you lose 0.05 seconds doing that, and the car lost potential being stuck on a higher than desired gear. If youre aiming for perfect corner, the car lost some of the balance because of the lift of the throttle.
-torque distribution
automatic is relentless in making the newb spin their tires unless they do alot of tapping. The cure is manualling. High torque cars need to shift to a gear higher than regular to obtain traction with the drive wheels. They can also stay on a higher gear entering a corner[this time its a good thing] because the torque is more than needed there already, and if youre on a lower gear as the fking automatic puts you, you'll end up spinning tires and losing traction + exit speed + maybe losing control of the car. an example is the shelby cobra, fully upgraded. With automatic, its a hassle to get this beast down the track and around curves. With manual, the monster torque can be distributed better, resulting in better track times of about 1-2 seconds and greater driver control + greater confidence into turns. Same with fully upgraded corvettes, and especially fully upgraded supras. Usually I just shift into 3rd for about all the turns in the course, and then use 4th or 5th on straights.
-powerband control
Why let the fking automatic shift at a 7000rpm redline, when the peak HP comes in at 5500 rpm?!? Case in point is the Plymouth Muscle Car, fully upgraded. I used to drag race a lot haha. Ok back to my point. With an automatic shifting at redline, the engine REALLY bogs down after 6000 rpm, and theres another 1000 rpm to go b4 the blessed shift!!! With manual, I can shift around 6000-6200 rpm and maintain a balance between the power going into the next gear and the loss of power on the current gear from bogging. Second case, dodge viper. The peak HP comes in around 6000 rpm, with a 5500 rpm redline. Why let the fking automatic shift at 5500, when you can get the max out of the engine by shifting at 6000?!? There are many more cars with lower redlines than peak HP, and you cant blow the damn engine in this game, so why fret, go for it.
-car control/training
it is easier to control the car once you get to use manual on it. You notice all the different spots along the rpm where things happen, like tire spin or a lot of pull. You also become a better driver[obviously...]. train yourself to listen to the sound of the engine and shift where you hear that distinct redline noise, or buzz, or watever you wana call it. The buzz for a type R, the roar for a mustang, same thing. Also, you can learn from listening how the engine should sound on any turn on a course. Like you would know what speed you are travelling in which gear and watever[i have no life, i do GT all day.....lol]
-drifting
also to do with tire spin on high powered drift cars, you can shift up to prevent over spinning the tires. Also, sometimes i found that i need to downshift during a drift, but the bastard automatic keeps me on a high gear so my car slows down to a stop and tires stop burning. Now if you use manual, you can downshift when you want to and keep the rubber hot!
-last but not least, the sound
i duno if im crazy, but i love listening to shifting gears because i kno i shifted it myself and not some crappy game program doing it for me. I also like the sound of engines exceeding redline too.
But simply put, automatic is a piece of crap... unless you are newb[great tool for newbs who haf no exposure to cars and/or manual transmission basics]