Those pipes are the ventilation system for all your drain pipes. Vent pipes allow methane and other nasty gases to escape out the roof, rather than inside your home similar to a p-trap. The ventilation system is there for all the sewer smells to go out on the roof. They are not a clean-outs; Some home owners believe that a plumber can use these pipes because it is a clean-out. It is true a drain cleaning machine will go in from the roof and into the drain system, but it will not work as well as a ground level clean-out. It is dangerous to try to clean a drain from the roof. Many plumbers used to do this in the old days, but with many accidents is something that many plumbing companies won’t do nowadays.