Oh, longevity, resistance to abuse...or even just ordinary wear. Especially if the shoe doesn't fit perfectly or the wearer has the propensity to walk the shoe out to one side or the other.
In the case of leather the strength is in the length and density of the fiber mat. With splits, the fiber mat is "ten degrees of separation" from the top grain and corium--the best part of a hide. It's less dense and more open to begin with, so there's little to prevent the fibers from pulling further and further apart until you have a hole over your protruding little toe, for instance.