Overview
Low denoise preserves the input but limits the degree of change. If the prompt describes a major transformation, a very low value may not give the model enough freedom to produce it.
High denoise increases variation but can destroy the composition or details that made the input useful. At 1.0, the workflow no longer meaningfully preserves the image.
The value is therefore not a universal quality setting. It is a similarity control, and the appropriate setting depends on how much of the source image should survive.