They effectively increase the power of the vertical stabilizer by preventing tip losses. Because the horizontal stabilizer on the Zenith is such a high lift device (downward) the gain is quite large.
The reason we use them is in extreme slow flight with reduced prop wash over the tail the increased tail force helps to keep the nose up for a shorter landings and takeoffs.
“I mounted the end fences with a bunch of Clecos and did some taxi testing. Wheely down the runway test..So with the end fences on I can get the nosewheel off the ground and keep it only about 6” off the ground at around 25 mph indicated. Repeated and got about the same. Then removed them and tried it again, it takes a lot more power to get the nose up then to maintain it I had to be going at least 32 mph indicated.”