There's a ball end on the slide lift arm. It slips in between two black (plastic? carbon?) pieces with concave ends to capture the ball. The top one is held down by the cap, the lower one is on a small coil spring so the pieces will move apart fairly easily. Just pull the ball out sideways you don't need to remove the cap. Getting the ball back in between the pieces is a bit tricky but isn't impossible.
Yes there are reasons for removing the slide other than cleaning, which is rarely if ever needed. Many mods - overbore, cam, pods, headers, etc - will need carb tuning. Tuning the carbs to compensate often requires moving or even reshaping the needles.
