Warns are used when there is a situation that requires 'long' preparation - that is, more time than a standby - I don't use standby's for anything longer than 45 seconds. Instead of saying "Hey Steve, you need to get ready for Deck 13" because Steve has to make sure X and Y are done, it's "Warning, Deck 13" (or "Warn, Deck 13"), then just before the cue - "Standby Deck 13". It is also used when a long run of cues are coming up - something like this:
"Warn, LX 13 through 15, Sound 12, Deck 1, Fly's 3 through 6, Standby Vision 1 through 3. Vision 1 .. go. Vision 2... go.. Vision 3... go... Sound 12... go... lx 13, fly 3... go..."