What is the primary reason for using a trap in a plumbing system?

The primary reason for using a trap in a plumbing system is to provide a water seal against sewer gases. Traps are designed to hold a small amount of water at all times, creating a barrier that prevents harmful gases, such as methane or hydrogen sulfide, from entering living spaces. This is critical for maintaining indoor air quality and ensuring the safety of occupants.

In addition to this vital role, traps also help to prevent insects and other vermin from coming up through the drains, contributing further to a hygienic environment. The water in the trap acts as a physical seal, effectively blocking these undesirable gases while still allowing wastewater to flow freely through the plumbing system.

The other options are not correct within the context of why traps are specifically utilized. While regulating water temperature, increasing water pressure, or controlling the volume of water flow are important aspects of plumbing, they do not pertain to the fundamental purpose of a trap.