The primary purpose of a P-trap is to:

The primary purpose of a P-trap is to prevent sewer gas entry into a building. A P-trap is a U-shaped section of pipe installed below sinks, toilets, and other plumbing fixtures. It holds a small amount of water, which acts as a barrier to block gases that can emerge from the sewer system, such as methane and hydrogen sulfide. These gases can be harmful and unpleasant, making the P-trap a crucial component in maintaining a safe and healthy indoor environment.

In addition to preventing sewer gas entry, a P-trap also plays a role in the overall plumbing system by allowing wastewater to drain efficiently while keeping the gases contained. This design ensures that the water seal remains intact, provided the trap is regularly maintained and not allowed to dry out. While a P-trap may assist indirectly in venting by allowing air to balance pressure in drainage systems, its primary and most critical function is to act as a barrier against sewer gases.