That's the spider's welcome mat. When a bug walks by, the spider feels the vibrations and pops out, injects a bite of venom, then drags the meal underground. All in the snap of your fingers. Other trapdoor spiders have even better-disguised doors. Surprisingly, they do just fine.
Prey routinely stumble too close to the opening, or even walk right into the spider's den.
It's a phenomenon that has not escaped the watchful eyes of arachnologists. Two phyla can play at this game. So one of the scientists' standard collecting methods is the door-less trapdoor. If need be. You can even use water, but it takes a while for some arthropods to drown, and you better check those traps daily, or the spiders and ants will begin dissecting one another.