A void type is used in a function to perform some side effects, such as logging; the built-in function pg_sleep is used to delay the execution of a server process in seconds:

postgres=# df pg_sleep 
                            List of functions
   Schema | Name | Result data type | Argument data types | Type 
------------+----------+------------------+---------------------+--------
 pg_catalog | pg_sleep | void | double precision | normal
(1 row)