It's basically just a hail verson of sand veil so it makes sense that it would make a user immune to hail ff it isn't naturally immune already. Another reason could be if somehow, say, a Venusaur, managed to somehow acquire snow cloak via skill swap or whatever, then that Venusaur would become immune to hail
Since it has been established that snow cloak is just a variation of sand veil, the other effects of veil is also carried over to cloak. For example, Cacturne is not naturally immune to sandstorm but thanks to sand veil, it is able to avoid sandstorm damage
In short, it is in the event a non ice type mon manages to acquire snow cloak through whatever means