Correct! That is indeed the purpose of the grooves. It acts as as a 'low-pass filter', allowing only low-frequency changes to the level of fuel inside the canister, which limits the float inside from sudden movement (hence maintaining a steady fuel reading).
The sender should be removed occasionally to clean out the grooves on the plastic disc as they can get filled with crud, preventing the gauge from draining properly and giving a false fuel level reading.
On one of my cars, I noticed all of a sudden that the fuel level reading had started bouncing erratically. On removal of the sender unit, I found that the bottom nut and disc had loosened and fallen off (and into the tank somewhere). This caused the float mechanism to react instantaneously to the level as the fuel sloshed around the tank (hence the erratic needle reading). I ended up having to drain the tank, remove it from the car, and turning/shaking it around until all the sender pieces had fallen out through the filler hole. Moral of the story: when reassembling the sender unit, always use thread-locker on the bottom nut that holds the disc in place.
Finally, be sure to inspect and/or replace the cork gasket underneath the sender (it only costs a few dollars). If the gasket is too old and crusty, your trunk will smell like fuel.
