[tracker1]

The canonical example for me, is when you want to store/use additional payment processor details for a transaction... If it's direct CC, PayPal, Amazon Payments etc. Relationally you only really care that the amount of the transaction was sent/received/accepted. But you may want to store the additional details, without a series of specific tables per payment processor. If you need to see the extra details that can still be done at runtime.

Another good example is for generalized classified ads, different categories may have additional details, but you don't necessarily want to create the plethora of tables to store said additional details.