The water softens and washes away black powder fouling quicker than pure Ballistol. Especially in the nooks and crannies at the base of the bore and in the cap nipple, without removing it.
It's actually not as good as getting it all out with hot, soapy water. But it does stop corrosion from happening if it gets left for a few days, before it gets cleaned properly.
Ballistol is great for coating the bore for storage, as long as it's just applied as a thin surface coat. The thing is, it should be removed with a solvent before loading and firing again. Ballistol does evaporate eventually and leaves a very sticky substance behind.