I'm guessing people do it to add character. Do you see anything wrong with this picture?
All I could see in this photo is electrical outlets that don't blend. Contrast is meant to bring out the features that are most beautiful. Although light switches, electrical receptacles and the like are very useful, they shouldn't be eye-catching.
In general, any sort of mechanical component should be painted to blend in. Gutters and fascia should be as close to the color of the house as possible.
A good place to use a contrasting color is a front door or a beautiful fireplace, not whatever this is below.