The electrical switches inside your car control functions throughout your vehicle. You have switches that turn your headlights and radio on or off, adjust the volume of your sound system, open the power windows, and lock the power door locks. While the functions you are controlling may be exposed to the elements themselves, such as the headlight assembly, the switches inside your vehicle are not designed to be waterproof.
Buttons, like the power window controls and the power door lock switches, are in close proximity to the window and can potentially get water on them if the window is left open. Manufacturers create their switches to cover the electrical contacts well so slight contact with water shouldn’t be detrimental.
The switches are not watertight, so prolonged contact with water may not only cause immediate problems, but also future problems through corrosion of the switch. Corrosion can form on the contacts, causing intermittent or complete failure, or it can form deep inside the switch. Also, the wiring to the switch may become corroded and need to be repaired before a new switch will work.
Some off-road vehicles, such as the Jeep Wrangler, have switches that are better protected from the elements. In some cases, the switches on these vehicles have a rubber boot to make them water resistant, though they're still not waterproof. This is not the norm in the industry, so protect your car switches from getting wet as much as possible.