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A downward slope is essential for proper drainage in plumbing because gravity plays a pivotal role in guiding wastewater away from fixtures and through the drainage system. When pipes are installed with this slope, it facilitates the flow of water and waste, allowing them to move efficiently toward the sewer or septic system. A properly sloped drainage pipe ensures that solids do not settle within the pipe, which can lead to blockages and backup issues.

While other factors like water pressure and temperature can influence certain aspects of plumbing, they do not directly pertain to the fundamental requirement for effective drainage. A flat horizontal plane would not allow for gravitational flow, and adequate temperature regulation is important for preventing pipe freezing, but it does not affect the drainage slope necessary for waste transport. Thus, maintaining a downward slope is critical for ensuring that plumbing systems function as intended.