What is a key characteristic of plumbing vents?

Plumbing vents play a crucial role in drainage systems by allowing air to enter, which equalizes pressure within the system. This function is essential because as water drains, it can create a vacuum that would impede drainage flow. By providing a path for air to enter, vents prevent such vacuum conditions and ensure that waste and water flow freely through the drainage pipes.

In contrast, insulation or placement below fixtures is not a requirement for plumbing vents. Additionally, vents are not directly connected to the sewer line; instead, they typically extend through the roof or outside to release gases safely into the atmosphere. Therefore, the primary characteristic of plumbing vents is their function of allowing air to enter the drainage system, making this the correct choice.