There are a few reasons someone might do a RTB rather than IB.
* If one of their search criteria breaks your house rules. e.g. My place is "not suitable for children" and the guest has a child or infant in their guest count.
* If your booking settings require positive reviews from other hosts. Guests with a review ❤️ stars (or maybe thumbs down in the review) can't IB and need to RTB
* If your booking settings require Verified ID, guests w/o verification can still RTB.
Basically, IB allows anyone with all your criteria to IB. If they're missing a thing, they can still request and you can override your settings by accepting them.