One of BRM's most powerful features is the in-built scheduling of your assets within your reservations. As you make rental tickets (or reservations) BRM auto-schedules each asset according to the dates of that rental ticket. Every asset (or item) in your BRM automatically comes with its own calendar, meaning that it can be scheduled with nearest-minute accuracy. This is a built in feature and normally you don't have to worry about it too much, it just works. It stops you making double-bookings for your assets.
If there is one thing however that is certain about the rental industry, its that 'stuff happens'. Sometimes you have a customer in front of you and you know you're going to put them on a certain bike (or bikes) and you just need BRM to take the booking. Perhaps there are other bookings that have been promised those bikes, or perhaps there are broken bikes that are preventing the booking working.
draft reservations solve this.Draft reservations let you save the reservation, and sort out the problems later. Perhaps you need to raise a support ticket for BRM support to investigate why you couldn't book. Or perhaps you need to juggle around some other reservations. Either way you just need to record the rental ticket and move on.
Of course you must be aware that this can be dangerous.
With drafts, you are specifically asking BRM to by-pass all of the usual checks and measures that the bikes are available. It is simply designed to buy you some time. The reservation will be marked as draft, and with a warning triangle saying there are issues. However if you overlook this, you can of course open yourself up to double-bookings. So use with extreme caution.
As a technical note, you will notice that if you look at the 'planner' (admin->show planner) for a given draft reservation; it will not contain any dates for your reservation. (though perhaps they might be taken by the problem booking which caused you to require a draft booking) This of course means the bikes are up for grabs from other reservations.
example show planner for a draft reservation: