I would assume it's like making pickles, or preserving organs/embalming a cadaver with formaldehyde. Well, the cadaver may be pushing it, but the other two are more similar. You are allowing the meat/ tissue of whatever you are brining soak up the mixture like a sponge and saturate throughout the item's entirety. This will change the flavor of it, and in some cases alter the appearance/texture/composition of the food (think about pickles and cucumbers.)
I guess you could say it's SORT of like marinading meat..... But not only are you using a more alkaline material (in this case saltpeter) to saturate the cells, you are letting it do so for far longer (whereas the above recipe calls for brining for 10 days, you usually marinate something for a few hours, a day or two at most.)