I asked someone who wrote a huge reddit post about it, and they responded with "idk, I just looked at it and didn't get it."
UI labs record a person trying to use something for the first time so they can see what they get stuck on. Like, mouse movements, clicks, even eye-tracking.
Not saying that the Lemmy or kbin devs should be doing that right now, as they've got full plates. Or that Reddit did this. But understanding where and why people get stuck is a big part of working on UIs.