What determines how well your body functions? The integrity of all the pieces involved. Forcing clients into multi-joint integrated exercises because an organization has labeled it as "functional" is a dangerous autopilot mindset that trainers need to get away from. This is incredibly difficult for most because it's comforting to have
a protocol rather than thinking through things or having to understand anatomy and joint mechanics. A client who may want to improve their ability to get out of a chair doesn't always mean starting with squats because it looks similar. Ankle dorsiflexion, knee extension, hip extension, and spinal extension are all necessary components to squatting. There's nothing "non-functional" about getting better at those. Stop dismissing single joint exercises because there's no rational basis to do so. This is all perpetuated by organizations selling certifications and blindly accepted by trainers who pay money to be associated with a team. If you decided you wanted to run a marathon you wouldn't start with one. You build up to it. Everything is a continuum. You must walk before you can run.