when to use KVO?

I have read many docs on KVO, but I'm still confused as to when to use it.

In case objA wants to monitor a certain property of objB, like so:

self.objB = [[ObjB alloc] init];
[self.objB addObserver:self
            forKeyPath:@"address"
               options:0
               context:nil];

so if objB's property changes, and it can only be changed by self, why not just do this:

self.objB.property = @"newValue";
[self doSomethingBasedOnNewValueOfObjBnewProperty];

instead of

- (void)observeValueForKeyPath:(NSString *)keyPath
                      ofObject:(id)object
                        change:(NSDictionary *)change
                       context:(void *)context

{
    if(keyPath == @"address") {
        [self doSomethingBasedOnNewValueOfObjBnewProperty];
    }
}

It may be useful when used with a singleton, like self.objB = [ObjB sharedInstance], where properties may be changed by other objects. Is this the only use case?