Short Answer
Siding is not generally intended to be the only line of defense against water. A properly designed wall assembly uses multiple layers and drainage paths.
Detailed Explanation
Siding is the first line of defense against water, but it's not the only one. A properly designed wall assembly is a multi-layer system: siding sheds most water, house wrap (weather-resistive barrier) provides a secondary water-resistant layer, flashing at windows, doors, and penetrations directs water away from vulnerable joints, and the wall assembly includes a drainage plane that allows any water that gets behind the siding to drain out at the bottom. This multi-layer approach is called a 'rain screen' or 'drained' wall assembly. The principle is that some water will inevitably get behind siding — through gaps, around penetrations, or from wind-driven rain — and the wall must be designed to manage that water rather than trap it.
Why It Matters
Understanding that siding alone doesn't keep water out helps homeowners understand why proper installation of house wrap, flashing, and drainage is critical.