So far as benefits in the garden, coal ash can help break up compacted clay, improve drainage and probably add at least small amounts of nutrients (although not as much as wood ash). ... The coal was mined from the Earth and burned, so it's akin to lime, greensand and similar minerals used in gardening.it's especially good to keep snails and slugs away; they don't like the gritty texture on their underbellies, apparently. Where I live now, it's too dry for them, so I don't have to worry about that.
I like putting ashes on the pathways in the snow; the darker color holds a bit of heat from the sun and the snow melts just a bit faster. The grittiness helps with footing too.