One way Valve

You can keep the filters and gun pressurized when you bleed off moisture from compressor so all you have to “recharge” is the length of line between compressor and check valve. At least that’s how I use mine.

To add to @DNF's reply
This also allows you to keep the desiccant under pressure when bleeding the compressor.
A practice that (from reading) I have learned is highly recommended when using a molecular sieve type of desiccant
And that is how I use it

In addition to the above posts , when topping off your tank from say , 3000 psi you can open the tank valve while the compressor spins up and the check valve will open when the compressor reaches the same pressure as the tank. Makes filling much easier as you don't have to watch the gauges before opening the tank valve.

I made my own check valve using a paintball tank fill ripple (male thread) and a female threaded foster fitting (matching thread to fill nipple). The fill nipple has the one way valve. It was WAY less expensive than buying a premade valve. You just need to make sure that there is a pressure release valve upstream and downstream of the check valve.

My check valve sits in the output of my second moisture filter. I put it in after having O-rings in the moisture filters fail creating a large air leak. Without it you need to shut the tank valve quickly and also turn off the compressor. You also need to let the compressor build pressure before opening the tank valve. With the check valve I do not need to close the tank valve if the filter springs a leak and I can open the tanks valve right after turning the compressor on. I could even open it first. It just makes things a little easier and helps avoid loosing air from your tank if there is an issue.

when you fill your gun direct,,, it has a check valve in the gun

well it is the same principle,, it holds the air in the tank when the manual valve is open on the tank
benefit on starting the compressor, open tank, start compressor, check holds tank pressure in the tank
benefit on shutting down the operation, quickly open bleed valve on compressor, let compressor run to cool down,,, close tank vlv and bleed hose by tank bleed vlv
first thing I bought when I started filling my own bottle,, number 2 was an ez fill

My configuration, takes a lot of air to fill the 3 filters, but can unhook everything and only lose air in 1st and last hose. Made 1 way valve going into 1st Centrifugal filter, 2nd has color changing silca beads, 3rd molecular sieve. Manual valve, with bleeder, can fill gun directly then disconnect. Gold value on tank was made as 1 way for just filling tank, but changed fitting so can fill tank, and fill gun from tank. Also use the tank to fill the filters some what before turning on compressor. It's a bit awkward but no matter how humid I'm getting absolutely no water in tank or gun anymore.