In addition to DVC holding a percentage of the rooms (as they are required to by law) there are also rooms that come back from people trading out points for things such as cruises or ABD trips.
When somebody trades in 500 points for a cruise, DVC does not say "that's nice", and send DCL the appropriate amount of money. Remember, these are separate business units, with separate accounting and budgets. DVC "pays" DCL, but then DVC wants its' money back. It gets that by moving those points into cash only rooms. If all the points rooms are booked, and something happens to one of them, DVC can move that person into a cash only room.
This is different than breakage, which is where there are points only rooms left over. During the breakage period, DVC will release these rooms into the available for cash bucket as well. However when these rooms are rented, DVC collects only the operating costs (the portion of the cash price used to pay for front desk, pool events, maintenance, etc) the part of the rental rate that is "profit" actually goes back to the owners of the time share. You will see this on your annual report as "breakage income"
This is why you sometimes see rooms available for cash and not points, points and not cash, or both.
If it is breakage, it will be available for both cash or points
If it is something that came from traded out points, where DVC is trying to recoup their money, it is cash only (unless they have to use it to cover a defective room)