Why or why not call an object with a “self.” prefix?

@interface{
   NSArray *array;
}
@property (nonatomic, retain) NSArray *array;
@end

@implementation
@synthesize array;
   self.array = [[NSArray alloc] init];
   array = [[NSArray alloc] init];

[self.array objectAtIndex:2]; [array objectAtIndex:2];
@end
@interface{
   NSArray *array;
}
@property (non