Some things completely change character after drying. Permethrin will kill cats while wet (like just after applying topically to a dog), but they wont absorb enough of it dry. And it doesn't really re-wet by just adding water (permethrin for hiking stays on clothes through 30 washes or something).

I'm not sure if it chemically changes or if it starts out in some kind of emulsion or something that can't be recreated by just rewetting with water.