More than once I've seen situation when there is a table, with serial primary key, and rows contain also some kind of creation timestamp, which is usually monotonic, or close to monotonic.
Example of such case are for example comments or posts in forums – each get it's ID, but they also have creation timestamp. And it usually is so that higher ids were added later than the lower ids.
So, let's assume you have such table, and somebody asks you to make a report on data from last month. How?