Being the main ingredient of most pets' urine, ammonia acts as a solvent, since like dissolves like. It's the only additive that can actually help getting rid of cat's spray.
I pour some in an empty spray bottle and saturate underarm areas of shirts well to remove antiperspirant stains; regardless of severity it always seems to work if let sit for a couple of minutes with maybe a quick rub. Never found anything to fade or get damaged, even when used full-strength on deep blacks.
Good additive to boost overall cleaning performance, but nothing that would sweep one off one's feet. Definitely good alkalinity builder for gentler detergents.