Following the rules
Tires are to be mounted with the label centered directly over the valve stem. Pro mechanics do it because it makes it easier to find the valve. You do this because that's the way pro mechanics do it. This will save you precious seconds while your fat ass sits on the roadside fumbling with your CO2 after a flat. It also looks better for photo opportunities.
Actually, I didn't do this - the guy in the shop who built it for me did. And the other tyre's exactly the same.
I have infinitely more respect for the place now.