The example I like to give is different payment types... CC/Payflow, Paypal, Amazon Checkout, etc... all you really care about is the type and amount and succes

The example I like to give is different payment types... CC/Payflow, Paypal, Amazon Checkout, etc... all you really care about is the type and amount and success... The additional details, different transaction id's etc are different... this allows one to use a single field for the "extra" details, while normalizing a few bits that may come back from different services in a different way, but a single type of collective data. Normalizing the above would mean at least 4 tables, with subqueries and joins that really aren't valueable at the database layer.