On some tractors, the only use for the hyd is for the 3pt. Pump, 3pt, tank.
The fluid goes from the pump to the FEL, and back to the 3pt. If you add more valves, the flow must pass through all valves in an open center system. Therefore a PB outlet is provided to pass on the fluid.
If you did not have a 3pt, the fluid from the FEL OUT port would go to tank.
The main reason for the PB is that a high pressure port is provided to withstand the high pressure when a valve downstream is activated. The cyl fluid is blocked from merging with the main hyd flow.
In a lot of valves, the OUT port can not take pressure above 500 psi.
Before you added the PB sleeve, the flow was from pump, to FEL, to 3pt, to tank.
After adding the 3pt sleeve, the flow is from pump , to FEL, to new valve, to 3pt, and to tank.
In a normal situation, with all valve levers in neutral, the fluid is flowing through all the valves with no pressure, just GPM's. If you push one of the levers on any valve and send fluid to a cyl, the hyd pressure starts to build when the cyl meets resistance, and will continue to build until the relief comes on and regulates the pressure. Any excess pressure is returned to tank.
If you activate the 3pt valve, the pressure is built up and reflected back through all the other valves.
On most valves, the fluid from the cyl the valve is controlling, flows through the OUT port and to tank.