what's practical example / use case SELECT SLEEP() for DBA
I have used it to create backups where I sleep until I can get a process ID from the processlist that is holding a database lock. I mentioned this idea back on Apr 17, 2011 in my answer to the post How can I optimize a mysqldump of a large database?

I wrote the answer to another post Jun 20, 2012 that has the actual code where I demonstrated the use of SELECT SLEEP (Safest way to perform mysqldump on a live system with active reads and writes?)

I have done things like this in the OS making physical backups using bash. I simply did the same in MySQL itself.