How many times do your customers ask for a portal showing sales or purchasing history of a product?
Or maybe an invoice history for a customer. In most of these situations, they want the portal sorted newest to oldest.
If there will only be a few dozen related records, then the fix is simple, just configure the portal to sort descending based on a field like Creation Timestamp or Serial ID.
However, when there are hundreds or even thousands of related records this can become very problematic and a real performance dog.
The same can be true of list views, or even sorted relationships.
Sorting is certainly always a red flag when it comes to performance as the data set grows.
This is even more pertinent with a WAN-first methodology.