Not a lot really. I am no grain scientist but here is the gist... Steeping, you are getting some sugar, color, flavor. A lot of typical specialty grains like caramel don't need mashed, as they go through a process before hand. For a mash You need a more controlled temp to convert the mashed grain (2row, pilsner, etc) enzymes to fermentable sugars. Temp is critical to the fermentability. You can successfully mash in a kettle on the stove (look up brew in a bag BIAB). It can be tricky with large malt bills if you have a smaller kettle.