The more you know about personalities, the better you will be able to understand why people do the things they do, and how to communicate with them. People who design advertisements are experts at how people think, and can come up with advertisements that help to sell their products. Society's leaders are experts at making people want to follow their way of doing things. If you understand other people, you are also more likely to get along with them and not have wars.
Studying your own personality is also good. The more you understand about how your mind works and how you think and feel, the better you are able to control your mind and not just react blindly to what's going on around you.