It started when I went (with Sawtooth) into the 40 Mile region for a high-ground caribou solo. Really bad storm lasting over 5 consecutive days with winds approaching 70 mph (per my Kestrel unit). I had the Saw nailed down with stakes and guy-outs. When the wind got really wild I carried big stones and put them on every last stake. When the wind got outrageous it was able to blow the bottom edges in and then the inside got breezy. Condensation flying...it was chaotic. I couldn't do anything to exclude the wind or pin down the lower fabric edges. I fought it off-and-on for 5 days. One day I was in the Sawtooth when a big blonde grizzly came into camp. Wind so loud I was unaware until I heard the bear vocalizing just outside. With the fabric edge raised by wind that bear could have stuck its big snout and head right in there with zero problem. Luckily he didn't. This is that bear:
Anyway...adding the sod skirt allows me to pin everything down with stones or moss. No wind ever intrudes. I suppose it adds a bit of protection from small critters getting in. With the sod flap inverted to the outside, snow and rain is directed away from the tipi edge. That pretty much sums up what I get from it.