Broadcasting, while it is one use of flash sms, isn't necessarily the only use.
I had a flash sms application for my Nokia E62 a couple years ago. Basically, it sent an SMS message from your phone to another recipient, except it popped up on the recipient's phone (this was mentioned above).
However, it also left no trace. A copy is not left in the recipient's inbox to view later or reply to. In addition, it was possible to add codes which changed the colour of the background and have flashing letters (hence the name, FLASH sms).
All that said, I know of no such applications for the Blackberry.
Here is the application I used on my Nokia. You can read the description to get an idea of what it's all about
: CoolSMS+ for S60 3rd : Nokia E62 (S60 3rd Edition) : Handango