Electrical outlets are usually placed approximately 15 inches above the floor, measuring to the bottom of the electrical box. This height varies between buildings and between electricians. Electrical boxes are almost never installed directly against the floor because they would be less conveniently accessible and would interfere with the baseboards at the bottom of the wall.
A height that is anywhere from 12 to 20 inches above the floor puts an electrical outlet in a location that is reachable but that can be easily obscured behind a bookcase or other furniture. Most people don't find electrical outlets visually attractive, and when they are in use they have cords coming out of them, making them even more visually disruptive. Keeping electrical outlets low on a wall also allows electricians to run wire at that height, making it less likely that a homeowner will put a nail or screw into the wire when hanging a picture.
One of the primary disadvantages of placing electrical outlets close to the floor is that they are then accessible to small children. Children can be injured or killed if they place a metal object into an electrical outlet. Most people with small children in the house install safety covers over outlets to prevent this from happening. Low placement of electrical outlets can also make them difficult to access, particularly when heavy furniture is placed in front of them. In very dirty or dusty households, outlets that are obscured behind furniture become filled with dust over time, and this can have an effect on their functionality.
The main exceptions to the placement of outlets at roughly 15 inches above the floor is in locations with built-in counters and furniture. Kitchens, bathrooms and laundry rooms often have outlets installed 6 to 8 inches above the height of counters. This makes the outlets easily accessible when working in one of these rooms but keeps them high enough so they don't interfere with the placement of a backsplash behind the counter. Outlets can also be placed in unusual locations in basements, workshops and houses where the outlets are retrofitted into the home after the completion of the original construction.