Why it matters: Users rely on the product’s/website's visual hierarchy and clues to know what to do and where to go. So it is essential to guide visitors properly using this hierarchy.
How to test it: Understand how the flows inside your product/website work and whether the key actions are being incentivised by visual hierarchy.
A good example:
Check this useful link:
Design Principles: an introduction to visual hierarchy
The theory of visual hierarchy is different from its practical application. More advanced concepts of visual perception are worth exploring because their mastery is key for great visual design.