What is the primary function of a drain trap in plumbing systems?

The primary function of a drain trap in plumbing systems is to block sewer gases from entering occupied spaces. This is achieved by creating a water seal within the trap, which effectively prevents the foul-smelling and potentially hazardous gases produced in the sewer system from escaping into the home or building. This water seal is crucial for maintaining indoor air quality and ensuring a safe living environment.

The drain trap must be properly installed and maintained to function effectively. If the trap dries out, it loses its seal, allowing gases to escape. This is why it's important for someone to regularly use fixtures connected to drains to ensure the water in the trap does not evaporate.

Other functions mentioned in the question, like increasing water pressure, filtering contaminants, or allowing easy drainage, are not the primary purpose of a drain trap. While some of these elements may be present in other plumbing components or systems, they do not pertain to the essential function of a trap in blocking gases.