What exactly constitutes a good return air setup, and how can one identify if their current system is underperforming in this regard?

I've been doing some research on HVAC systems lately, trying to understand how to optimize my home's heating and cooling efficiency. One thing that keeps coming up is the 'return air' system, and honestly, I'm a bit confused about its true importance. I understand it's about bringing air back to the furnace or AC unit, but why is it so critical? Is it just about circulating air, or is there a deeper, more fundamental role it plays in temperature regulation and overall system performance? For instance, if I have a room that's consistently warmer or colder than the rest of the house, could it be an issue with the return air in that specific area? I've heard some people talk about 'return air imbalances' and how they can lead to significant energy waste and discomfort. What exactly constitutes a good return air setup, and how can one identify if their current system is underperforming in this regard? Are there common misconceptions about return air that lead to problems? I'm particularly interested in how a well-designed return air system contributes to even temperature distribution throughout the house. Does it directly impact the sizing and efficiency of the main HVAC unit itself? Any insights or experiences would be greatly appreciated as I try to wrap my head around this crucial component of home climate control.