I like it for the simplicity and because of what was mentioned above.... dirt. My super tarp/bivy setup has twice the room of equivalent pup tents and at half the weight. I can crawl underneath in a downpour with muddy boots, change clothes, then setup sleeping bag and everything that was dry remains dry, everything that was wet can stay on the other side of the tarp. I haven't used it as much now that I live in Alaska (summer weekend hikes mostly), but in the right setting floorless is amazing. I have been using the Kuiu Stormstar this fall for sheep hunts and so far I love it as well. Heavy for it's size, but closer to Hilleberg strength, I LOVE that it pitches with the tent already inside the rainfly so your tent doesn't get soaked as you rush to put on the fly.
The simple answer for why to go floorless is easy, the weight tradeoff (when weather allows).