在Xcode 4中使用@property时，为什么要在@interface中声明对象?

@interface first : <NSObject> {
   NSString *myStr;
  /**
       I don't understand why declared this NSString *myStr.
       Even if not declare NSString *myStr, this codes work well.
  **/
}
@property (nonatomic, retain) NSString *myStr;

and Add @synthesize to first.m
@interface first : <NSObject> {
   NSString *my