The goal of project controls is to link planning and execution
In my experience, project managers love to plan. It can almost be too much fun. I know I can sit for a day and build a massive gantt chart with a complex critical path
But then you get to construction, your phone starts going off every 30 seconds. Everything else goes out the door and all you do is focus on building.
You very quickly lose all the benefits of building the plan in the first place
Project controls is the discipline that links planning with execution. it keeps your plans relevant and useful
Check out this video that goes into detail about the fundamentals