How to change the context of a function in javascript

I'm trying to understand why in javascript, you might want to change the context of a function. I'm looking for a real world example or something which will help me understand how / why this technique is used and what its significance is.

The technique is illustrated using this example (from http://ejohn.org/apps/learn/#25)

var object = {}; 
function fn(){ 
  return this; 
} 
assert( fn() == this, "The context is the global object." ); 
assert( fn.call(object) == object, "The context is changed to a specific object." );