A dehydrator is used to...... dehydrate the nail plate... remove oils from the nail plate... so that the product you are about to apply has the best chance of adhering to the nail plate.
And an excert from Schoons book
Nail dehydrators (Prep) are extremely important. Moisture can interfere with the adhesive bond which leads to delamination or lifting. To ensure proper adhesion, always scrub the nail plate, dry thoroughly and dehydrate. Skipping one step will lead to delamination and may also contribute to infections. Some preparation products contain pathogen fighters that remain on the nail plate to aid in the prevention of nail infections. The effects of dehydration are apparent for up to 30 minutes before the natural moisture levels are restored by the body."