* To stay on the nails: If nail varnish dissolved in water, it would wash off every time your hands got wet. This would make it incredibly impractical and useless as a cosmetic product.
* To protect the nails: Nail varnish acts as a protective barrier, preventing the nail from becoming brittle or discolored. This protective function relies on the varnish remaining intact, which wouldn't be possible if it dissolved in water.
* To maintain its color and shine: The pigments and resins that give nail varnish its color and shine are also not soluble in water. If they were, they would wash away, leaving the nails dull and colorless.
In summary: Nail varnish's insolubility in water is essential for its functionality as a cosmetic product. It allows it to stay on the nails, protect them, and maintain its visual appeal.