Back in 1996 GM started using roller cams in their engines, those same roller cams hit the marine market a year or two later. When roller cam engines first came out there were all sorts of exhaust reversion issues because boat builders used a shallow static water line measurement. Those check valves act as kind of like an idle relief valve. They take some of the back pressure off of the exhaust system, and it theory, was supposed to help with reversion issues. They never really worked. If you go through some of the old installation books you would see that the static water line measurement was something along the lines of 9 to 10 inches. Volvo then changed to something like 14 inches a few years later, and did away with those check valves. If you have them, check them for signs of water. If you are seeing any water in either of them the risers are rotting. Other than that, don't worry about them or just pull them and stick brass plugs in the risers.