The Fresnel channel highlights the areas in the image where the Fresnel effect is more patent, mainly those areas at glancing angles in surfaces with low Roughness (specular surfaces).
This information is useful for compositing purposes, i.e. when you need to select the areas at glancing angles to enhance their reflections or tinct their reflections.
The render (left) and the Fresnel channel (right). Note how the highly specular surfaces and the glancing areas (i.e. in the glue joints) are highlighted