I've met people in my career I can instantly classify into a certain personality type: she's a bulldozer, he's a cloud-thinker, she's an emotional vampire, he goes for shock value, and so on. We categorize others automatically, sometimes without even knowing it as it helps shape our response and communication. Ever notice the shift in communication styles and personalities when the CEO walks into the team room; people sit up a little straighter, talk with more meaning and direction, and appear busier all of a sudden. Effective leaders recognize personality styles in others, as well as in themselves, and use this information to shape the ways in which they interact and lead.
I'm sure you've seen the Myers-Briggs and Jung, and Kiersey models that focus on some aspect of extraversion/introversion, sense/intuition, think/feel, and judge/perceive. A basic knowledge of your type can help you figure out why you may react a certain way in certain circumstances...