To limit the probability that roof sheathing will not be installed under valley flashing, which could lead to insufficient support of the flashing, which could lead to excessive deflection or failure of the roofing.
This is to limit the probability of compromised structural integrity, which could lead to precipitation ingress, which could lead to deterioration, which could lead to compromised structural integrity of roofs or assemblies protected by roofs, which could lead to structural failure, which could lead to harm to persons.