Getting back to basics, plumbing vents are the little stacks that stick out of your roof. They allow water to flow out of your drains by displacing it with air. If you didn’t have them your toilet wouldn’t flush or your shower wouldn’t drain. Since they connect to the sewer system under your house, smelly and explosive sewer gasses my be present, so they vent to the outside. Thus you have the little stacks. Since they are on the roof and lead directly to your toilets, this is how animals such as frogs and rats get to your toilets. Critter Quitters will stop this and protect your vent to boot.