[Gpetrium]

Companies and employees need to be able to sell the idea of a project/product/service to each other. The waterfall method serves as a bridge that connects the dots, allowing both sides to understand how they intend to reach the end goal.

The same way you haven't seen waterfall in its pure form, you will likely never see Agile, Design Thinking, etc in its pure form. That is because we are looking at environments prone to change and in control of humans with variable degrees of knowledge and prone to change.

What you have seen with the diagram that shows arrows in all direction is simple, it went against your perceived view or the standard and that put you off. Or, the person just didn't know how what they were doing (the more likely scenario).