OK this is going to sound really stupid but bear with me
When I was first starting out and getting into mucking around with air systems and markers I had the whole thing explained to me like this and it's pretty simple - think of water
The water behind a dam - this is your air at 4500
The dam controls the flow to a "reasonable rate" - this is the primary air reg (e.g. Melee) 4500psi down to 800psi
The Water company controls the flow to your house - this is the secondary/inline reg into the gun - e.g. 800psi down to 300psi
You turn your tap on or off when you need it - this is the valve in the gun allowing the 300psi air to shoot the paint.
I know it sound stupid but it does fit