What is the main reason behind having a lot of horsepower in a car's engine?

What is the main reason behind having a lot of horsepower in a car's engine?
There really isn't a purpose. They use it for people such as race car drivers to provide speed, but the common person doesn't need it. Though some people like to say, "Yes, my car has a lot of horsepower" to sound cool, but it just provides speed.