When water runs down your drain, it displaces the gasses that are located inside of the pipes. Usually, they are funneled out of your pipes through a vent pipe. The reason the sewer gas doesn’t come back out of your sinks is that they have p-traps installed. You will recognize a p-trap as a “P” shaped pipe underneath the sinks in your home. These block out sewer gas by retaining enough water to prevent passage through the pipe. This does not impede water flow through the pipe but functions as a good seal. Sewer gas is not just foul-smelling, it is also has a negative effect on your health. Over time it can lead to serious health side effects, so you can appreciate how important a p-trap is. Appliances that need to drain water will be connected to your sewer line, so they will need to have a p-trap somewhere along the way. Some of them will have a p-trap built-in, while others require you to connect them to an existing p-trap.
