Understanding air flow on compressors

For longer than I should admit, air settings on air operated tools and compressors has stumped me and I'm hoping I can start my education today and stop being ignorant of air

We have various devices that require air, HVLP guns, Sand Blasting cabinets, etc.

The part I don't understand is the things people require. For instance, our Sand Blaster, we might be blasting at 20 psi (detailed work on glass). The regulator at the sand blaster is set to 20. However, the specs from the manufacturer say that you need no less than 90 psi coming into that fitting. Why do I need 90 psi coming into a regulator that is going to throttle it back to 20 psi? The compressor has a large tank, so what's the need for it? And if memory serves me correctly, if you do turn the compressor down to 30-40 psi, the sand blaster won't work.

On HLVP guns, they say that your gauge at the gun needs to be 25-30, so that it is 12-15 at the actual tip. However, if I put the compressor at 90 psi and the regulator a the gun to 25, then it's WAY too much pressure. If I scale back the compressor to about 40, then it all works fine.

It seems to me like there is a direct relationship between the air pressure on the compressor and what's coming out there other end, even though there are regulators in place.

In my mind, if I have a compressor and it's set to 100 psi, and then I have a device with a regulator on it that's set to 10 psi, if I am discharging 10 psi out there device, and I walk back to the compressor and turn it to 25 or 200, it shouldn't change the air output of the device one bit. However, I don't think that's my experience.

Anyone want to take on the challenge of trying to help me understand that relationship?

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Understanding air flow on compressors