Some RDBMSses have the possibility to sort tables (next to indexes of course).

For list functions this can be beneficial. For example by sorting readers on postcode so the readers of one area are grouped together in one block.

an unsorted table a sorted table
Unsorted, the system must hip hop through the whole table to find the related rows. Sorted. The system just needs to fetch one block. The rows are grouped together.
Search the documentation of your RDBMS for clustered index.