A Real World Example
One of the most-often requested features is the ability for the store to automatically add a free gift to the Shopping Cart if the customer spends more than a certain amount.
The code has to be intelligent: it has to not only add the free gift when the shopper spends over a certain amount, but also remove the gift if the user changes the contents of the shopping cart, such that the total falls below the threshold.
Traditionally, although the code for this is not particularly difficult, it would have meant 'hacking' the core shoppingCart class in a number of places. With the ONS, this can be achieved with one very small custom class and absolutely no hacking whatsoever.