Pipe Under Roof Eave
This pipe is actually a gas pipe that is running
around the perimeter of the house underneath the roof eave.
Someone installed a hot tub in the backyard and chose this as the easiest route to run the new gas pipe.
Most gas pipes in a situation like this are ran underground.
Local building departments allow plumbers to install gas pipes this way most of the time. The only problem is that it doesn't look nice to most people.