I was packing the dome and all the other things into my carry-on bag for the flight to Bonaire, and felt that the things that were going to be above the dome in the bag might bang into it and scratch it, even with the neoprene cover. What went on top of my dome in my travel case was the macro port, in bubble wrap, and one strobe on top of that, so it wasn't a huge load, just not one that I wanted putting any pressure on the port itself.
Dust and sand could get on the cover and get ground into the port, and that would be in the center of the port, easily seen. The 10.5mm has such depth of field that dust inside the port and scratches either on the inside or outside of the port could show if if is stopped down enough and focused on a pretty close subject.
You sure could use cardboard to cover the dome for the flight, but that might also get pushed in over time. I had read somewhere that another photographer had adapted a piece of tupperware to cover his dome, so I was searching around in the kitchen, holding my dome port when I came across this basket for the rice cooker.
An additional bonus is that I can use this helmet in the field, going from hotel to dive site. It protected the dome when it was in the back seat of the pickup truck as we bounced around Bonaire. I would remove the helmet and put just the neoprene cover back on before I entered the water. After I exited the water, I would put the helmet on underneath the neoprene cover before driving off.
I still got scratches on my dome that I have to polish out, but that was from swimming into some coral (DOH!). Scratches that happen underwater are much easier to forgive than scratches that happen on land. I just didn't want to arrive in Bonaire with a scratched up dome port to begin with!