Depending on your needs, your internal speakers can be more than enough. Most computer models these days offer superb audio systems, eliminating the need to buy external speakers. Internal speakers produce decently crisp sounds that are great for casual gaming, movie-watching and even voice.
But if you're into music, gaming or sound production, PC speakers might be a better option. They can enhance your audio set-up and provide richer surround sound. Better sounds can create a better user experience as well.