Re wet clutch.
IMHO the main reason for the exsitance of the wet clutch is size and weight.
Some vague memories of wet clutches
All the wet clutches I've worked on, all Jap bikes, were multiplate. The clutch cage has as its base a large cog. This is 5 to 10 inches in diameter and is driven directly by a small cog on the end of the crank shaft. Slotted into the clutch cage are pairs of plates, one asbestos and one metal. Some clutches have up to 8 pairs of these plates, e.g. a trials bike, but 4 or 5 pairs are more common. The asbestos plates locate with the clutch cage so that they spin with the cage at all times. The metal plates do not locate with the clutch cage. The metal plates do locate with the clutch centre which is on the end of the primary gear shaft. Once all this lot, with rubber spacers etc is in place the outer piece bolts onto the clutch assembly with 6 or eight bolts with springs under them. Note that these pieces are marked as they must be assembled with the correct orientation. The springs compress the pairs of plates together so that when the crank turns the clutch turns the asbestos plates turn the metal plates turn clutch centre which turn the first gearbox shaft.
Through the gearbox a thin metal rod is linked so that when pushed it forces the clutch centre out away from the clutch cage, pushing against the springs. This allows the asbestos and metal plates to move apart and hence allows the clutch centre to spin at a different speed to the clutch cage.
The whole lot is bathed in engine oil to stop it sticking and help cool it.