It’s a common saying…but how true is it?
I always thought it just meant water was a good thing to have.
My friend has heard that snow delivers nitrogen to the ground; the idea is snow traps air (mostly nitrogen) as it falls, and when it melts it somehow drags the nitrogen along with it into the ground.
Some websites (http://www.weathernotebook.org/transcripts/1999/04/26.html) seem to support this.
But I suspect this is not so; when snow melts into liquid water any trapped air is released. There may be some chemical impurities in the snow due to pollutioin, but that would likely not be beneficial to plants.