That is good news that you've never had to use it, this means you can probably turn pretty close to 90* while backing up without a problem with your current setup.
The times that you may want to use the slider is when backing up in a tight spot on uneven ground. The uneven ground can make the nose of the fiver tip closer to the truck cab, so even though the turn may not be any sharper than what you've done in the past, a dip in the ground where the truck's rear tires are, or the fiver tires go over a higher ground, can bring that fiver nose close to the cab, and then wack! Pretty glass bits everywhere.