Water spots on a car are caused by the minerals and dirt left behind when water evaporates on the vehicle surface. The spots can appear as circular, cloudy marks, and are common after rainfalls sprinkler contact or improper car washing techniques. The primary components of these residues are usually calcium and magnesium, and they can etch into the cars paint, if not cleaned off promptly.

Here's why it's important to remove water spots

Aesthetic impact water spots can significantly mark the appearance of a vehicle, making it look dirty and poorly maintained, even if it is otherwise clean.

Damage to paint overtime the minerals and water spots can corrode or etch the surface of the paint. This etching can be especially severe if the water is particularly hard (high mineral content) or if the spots are left to bake under the sun which can intensify their effect. 

Impairing visibility  when water spots form on windows and mirrors, they can impair the drivers visibility, which is a safety concern.

Resale value just like with other forms of neglect failure to remove water spots, can lead to deterioration of the cars exterior which might reduce its resale value.