Good question.
Well, I first made an inverted coke bottle trap but it couldn't catch a thing. Then I added the ramp and the trap caught everything. I could have just gotten another bottle and built another trap that just had a ramp. But, I didn't have another bottle handy.
However, with this design, I think maybe the wider opening at the top allowed me to more easily control the water flow in and out of the trap. By placing the trap under the aquaclear, I can see small particles move out of the trap and down the ramp. That's when I see the inverts notice the bait and climb up the ramp. If there was no water flow through the trap, the inverts don't seem to notice/smell the bait.
So no real good reason for inverting the bottle other than I already had it that way and maybe it might help let the inverts smell the bait as the top acts as a funnel for waterflow.
By the way, the ramp was just the plastic tube that holds the coral epoxy sticks.