Yesterday's ARCA race at Daytona International Speedway had a peculiar thing happen: the blue flag accidentally fell out of a flag stand onto the track, which caused a yellow flag in itself.
The best explanation I've heard for the blue flag I've ever heard is "there's a race going on and you're not in it." It lets a driver know there's a frustrated faster driver hanging off his back bumper that needs to get by. It's an important flag.
Given ARCA's tendency towards big pile-ups like this, flags are kind of necessary to keep in the stand:
Even the one for slow traffic is needed, particularly when someone tries limping a busted car back to the pits.
Unlike many road courses where the flag station is away from the track a bit, this flag station hangs over the track itself at Daytona. There is no opportunity to send a guy down to fetch what you've dropped.
Fortunately, one of the cars whizzing through seemed to have picked up the flag for them. Hopefully they had a spare in the stand itself.