Delegates in sapui5

Hi,

what is the use of event delegates in sapui5.? I have seen some examples where they use

this.addEventDelegate({

onAfterRendering : function(e)

{

// some logic

}});

My question is already in the controller we have onAfterRendering hook method which we can make use of, why to explicitly use delegates? I am a  newbie please kindly help.

Thanks

Sushmitha