Red is most often used to symbolize excitement, energy, and passion. In Japan, red is seen as the color of happiness (hence Nintendo, a toy and games company, using red on their logo and website).
Some sources state that the color red can increase respiration, blood pressure, and metabolism. Red is also synonymous with power, so it can give something an air of importance or officiousness.
Companies often use red because its intensity creates a sense of urgency that can make consumers more likely to look at a product; this is why it's often used to highlight sales in stores.