Basically performSelector allows you to dynamically determine which selector to call a selector on the given object. In other words the selector need not be determined before runtime.
Thus even though these are equivalent:
[anObject aMethod];
[anObject performSelector:@selector(aMethod)];
The second form allows you to do this:
SEL aSelector = findTheAppropriateSelectorForTheCurrentSituation();
[anObject performSelector: aSelector];
before you send the message.