Functional requirements detail the behavior of the intended system. This is from the workflow to the end-user interface. You want to ensure that each functional requirement that you capture is just that.
Non-functional requirements are the requirements that stakeholders and users haven’t thought of, but you have to because without them, the system will fail. If you don’t collect non-functional requirements, then you will not be creating a system that behaves in the way the users need it to.