A simple way to understand it is to think of your OBB placeholder as an actual reservation - except without a sailing or room attached to it. Let's call it Booking A.
When you book online on your own, you create a new reservation on a specific sailing and in a specific room. It's an entirely new booking, and let's call it Booking B.
To attach this sailing and room to 'Booking A' later, you need to release them from 'Booking B' first. And the only way to do it is to cancel 'Booking B' -- since you as an individual can be on a specific sailing only in one instance.
And, as highlighted by others, this process of cancelling 'Booking B' and modifying 'Booking A' runs the potential risk of your losing that room altogether if someone else snaps it in that split moment. And you will also be subject to the new pricing at the time 'Booking A' is modified.
I would say that, in 90-95% of the cases, the risk of losing the room is very low if the agent on the phone is experienced enough.