Sometimes, you have to delete a lot (and I mean a lot) of message at once. That's where
Note: this requires you to be an admin in that group!
delete all messages
If you reply with
/purge to another message, this will delete all messages which have been sent after this certain message.
delete all x messages
Just reply with
/purge <int> to another message, where int is any number. This will delete as many messages as you have specified.