Delta Classic Tub Shower

I am installing a Delta Classic Tub/Shower. The instructions call for a minimum 8 inch long pipe (1/2 " thick copper or iron) to be used for connecting the valve body to the tub spout. I didn't realize it called for 8 inches and the one I installed is about 5 inches. Why would it need to be 8 and what might happen if I leave it at 5. Would there be any issues?

If the tub spout cannot discharge sufficient water, it may start to come out the shower head

I can't see why the length of the pipe would effect the amount of water it would discharge?
I would assume it would be so you dont shortchange yourself and realise you dont have enough pipe to work with.

I don't think anyone on this forum had any input on the design spec of that Valve.
Therefore their intent to put a minimum length is unclear, because they don't say why.
Any answer would be pure speculation.
Longer than 18" and more than one 90 could cause excessive resistance and cause water flow to turn back toward the shower riser.

I personally don't know why a shorter drop to spout would make any difference.

My speculative answer would be that it would just be odd looking to have the spout so close to the valve that it would look like some hillbilly install and being so close to the spout, possibly interfere with it's operation

Is it for trim ring and spout clearance