Property flags in JavaScript are used for defining and controlling property behavior and characteristics: 1) They determine how properties can be accessed, modified, or enumerated, 2) They provide fine-grained control over object property behavior, 3) They include writable, enumerable, and configurable flags, 4) They help in creating objects with more specific and controlled behavior, 5) They support information hiding and encapsulation principles, 6) They're fundamental to implementing advanced patterns like immutable properties, non-enumerable metadata, and defining custom property behaviors.