This is a WARNING.
On this site, there are no dumb questions......other than the ones you don't ask and should have. :sidelaugh: :sidelaugh: :sidelaugh: :sidelaugh: :sidelaugh: :sidelaugh: :sidelaugh:
The holding valve is essentially a check valve but it is more aptly called a pilot operated check valve. Imagine for a moment that you installed two shut-off valves on the lines that feed the motor (one valve per line) and you joined the lever handles together that control the two valves and put an extension rod up in the air so you could operate those valves at will
If you came to the top of a hill and started to descend it, you could pull on the rod and shut the valves off. This would prevent any oil from getting in or out of the drive motor and that would cause the motor to hydraulically lock up. Now, if you pushed on the rod a little bit to cause both valves to open a crack you could control how much fluid enters and leaves the motor. This would give you hydraulic braking as the tractor descended the hill.
In essence, that's what the holding valve does, however there are tiny pilot valves inside that detect incoming oil pressure and that pressure on the pilot valves causes the larger valves that control the in/out oil to the motor to open up proportionately. This valve keeps the tractor from running away on steep grades.
Does everyone need one? No. Not if you rarely encounter steep grades. Are they nice to have? Yes but they are not essential.