There are a number of advantages...

Trapezing from the raised rail(as on a 16) is easier than from the hull. Your feet are above the wave action that sweeps along the hull. Less getting your legs washed out from under you.

Trapezing from the rail is drier.

Carpet or neoprene on the rails gives better traction than most hulls. Better footing.

There is another large advantage to the raised trampoline frame that would only be noticed by those that sail both... In a cat with straight crossbars mounted directly to the hulls (not arched like the 17, 18 and Hobie 21) you will find that the boat is less comfortable to sail in rough conditions. Especially evident downwind. With the low crossbars, the wave action rides up under the trampoline and when it hits the rear bar, the boat decelerates suddenly and the rear is pitched upwards.