Allowing scaffolding to sit on trapdoors is extremely important in redstone as they update with a 1 gametick delay, instead of a 1 redstone tick delay (2 game ticks). Allowing their base to sit on a closed top-half trapdoor but not on an open top-half trapdoor allows redstoners to create a one gametick (or 0.5 redstone ticks) delay, (which is otherwise nearly impossible with the base components, which update either instantly or after 2 game ticks)

It is currently possible to do replicate the delay with a sticky piston, however this still creates significant issues such as not working with a one redstone tick pulse because sticky pistons will spit out their blocks at that point