system
A chat on subreddit /r/pcmasterrace
human
What situation is that mode desirable for then, since you can't line up your gpu to the case slots ?? I assume there is a good reason why lian li made this, but I can't figure out what that reason is.
gpt
There are two situations that I can think of, The first possibility, though less common, would have to do with the motherboard but I can't think of any model names, and google isn't being very helpful right now

Iirc there is a very very small amount of motherboards that have more than one "main spec'd GPU slot" but they are stupidly expensive, I think the most recent post on reddit I have seen about one was an AMD board (I dont remember how long ago though), and in the case of those boards, since they have more than one full speed x16 slot, they would line up properly in "Aircooled Mode"

The other situation would be if the user were to vertically mount their GPU in the case with a PCI extension ribbon, If the GPU is mounted in the Vertical slot instead of the Horizontal slots the location of the PCIe slot would not matter as the GPU wouldn't need to be directly plugged into the motherboard slot