IMO the relaxed span is used for casual bowlers, bowlers who only bowl recreationally. Since I still have a coupla balls with relaxed spans...from back when I first started bowling....I recognized that it is hard for me to be consistent, throwing these balls.
My hand can too easily clench, cup, or loosely hold the span holes, therefore not affording me the defined "feel" needed to get proper muscle memory going, while trying to repeat shots.
On the stretched span, I immediately know the grip "feel", and can alter or adjust my releases based on that benchmark, starting point.
Hope that helps.
--------------------
Duct tape is like 'The Force'. It has a light side and a dark side, but it still holds the universe together.
Some days you're the bug; some days you're the windshield.