Minecraft wiki entry for Snow_farming, associated text: To combine with a pumpkin farm, allowing the construction of a huge army of snow golems, which can be used to either throw snowballs at mobs, or to produce more snow.
Snow blocks are a decent building material as long as they don't have to resist explosions; contrary to all logic, they don't melt. Thus, an easy-to-build snow farm (and a material to make lots of shovels) can avoid the need to haul many stacks of cobblestone or other material to a build site for constructing large farms.
Hostile mobs cannot spawn on snow layers stacked 2 or more deep, if light sources aren't desired for a build.
Endermen cannot teleport onto snow layers stacked 10 or more deep.
To use as weapons against blazes, saving the durability of your sword or bow for other mobs.