As said, it was a feature introduced to help with an issue they had where if the bar was left in place, then the endless jiggling about lead to wear of first the securing pin, follow by the tange/receiver. This caused towing "knocks" and these further damaged the receiver.
The ratchet moulded in the green knob helps stop any unwanted loosening of the locking pin.
If you buy a D3 with a bar featuring the smaller ratchet less knob get LR to check the fit with their test gauge, just a few minutes job and was a "recall" . Remove it when not actually in use, and listen for any knocking in use.
With moderately fretted receivers, LR change from the removable to their fixed tow bar, with badly fretted receivers, they cut out and replaced the crossmember.
I had the knocking with our D3, and LR retro fitted the fixed one, I did though tend to leave it installed between jobs. I have no issues with the D4 and later plastic ratchet type removable towbar, but it has never done any miles installed and not in use..