The pre-booking message is used only with instant book. When a guest starts the process, and requests to book, they will be sent this "pre-booking message" before confirming their instant book option. According to Airbnb's informational pop-up associated:
Pre-booking message
Guests see this message before booking. This is an opportunity to say hello and add any final details they should know.
Here's what we say (you have a character limit):
Thank you for choosing us. Please read the house rules carefully. Once you've booked and are ready to travel, we look forward to getting to know what's bringing you to Durham. Let's keep in touch!
Without the opportunity to go through the Instant Book process without actually booking something, I'm trying to recall when this message comes up in the process but can't.