We currently have three contracts, two with one UY, the other with a different UY. Our third contract is small, so it's really not big enough to make a good reservation every year. So we use our once a year transfer to transfer those points into our other UY. The third contract is the same resort as the first contract (the second is a different resort - have you got all that ?? Phew!!) So we can then combine the points for all three at 7 months or combine the two at 11 months.
If we didn't transfer every year, we would only be able to book as many nights as we could with that small contract and link the reservation to any we made with the larger contract. You can't combine points for a single night between two different UY(Master contracts).