is there an advantage of using manual clutch? in terms of race driving. like in traction control, you get benefits of turning it off.
It used to be possible to change gear much faster, in GT5

particularly, by riding the clutch (well, you disengage it immediately, and the engine / drivetrain ride it) instead of waiting for the game to rev match. Also, flat-shifting (not releasing the throttle). It was heavily nerfed, though, in that flat shifting was eventually disallowed, and you cannot engage a gear if the driven wheels are spinning (that might be a physics issue), nor can you change into second gear in less than a certain amount of time (for standing starts; first and second tend to be wide apart on most cars, so the "clutch riding" is most advantageous here, where the sequential boys and girls have to wait forever for the revs to match). In GT5, gearchanges are much faster overall, so there's even less of an advantage. There's also the disadvantage of mis-shifts, heel and toe requirement etc. with H-pattern / clutch.
Anyway, there's no point offering the clutch to everyone whilst the drivetrain physics are so obviously broken. The clutch physics, too, are not physically based, in that slip and torque transmitted are not correct (the clutch slips, but transmits less power, otherwise the revs won't drop...).
Try this: at a standstill, put the clutch in (gearbox in neutral) and rev the engine up high somewhere and keep it steady. Then let the clutch back out. The revs should drop slightly as the clutch transmits the rotational speed of the engine to the input shaft (and the gears themselves) of the gearbox, which would have been stationary. Similarly, the engine should rev more freely with the clutch in, because it's not having to turn the gearbox.