In What Situations Would You 'Pass By Value' And 'Pass By Reference'

I have been trying to grasp the whole 'Pass By Value/Reference' thing for a few hours now. From what i can make out:

Passing by value will make a copy of the value, pass it as a function argument and the function will store the changes, the original value is not affected. So for example:

JavaScript Code:
var cost = 145;var postage = .3;var a = function() {return cost + postage;} //Using an Anonymous function for this

If you was then to pass the value of 'a' to a function argument it would copy and not change the original value.

If you pass by reference it will change the original value. When you pass a reference to a value through a series of different functions and the value is constantly being changed the change happens on the original value and can be seen outside the function.

The problem is i cannot think of how or why this would be used. I havn't got to develop many large applications so im trying to think of a few situations when passing by reference would be used.