The swelling of gelatin in cold water followed by a melt serves two functions:
1. Directly mixing gelatin in hot water introduces air bubbles due to the agitation of the mixture. Heating a swollen gelatin to a liquid state allows the mix to degas.
Commercial film coating is typically accomplished without this swelling stage. Heated mixing kettles capable of holding 750 -1500 liters feed into devices that filter and degas the mix.
2. The bonds of the polypeptide chains formed by a swell/melt gelatin mix are somewhat stronger than a directly mixed and heated one. This is probably more important for alt processes that rely on cross-linking for image formation.