Each owner is given a certain number of housekeeping credits based on the number of points they own and are allocated for a given use year, and it requires 1 HK credit to make a reservation. If you deposit points to a future use year, the HK credits do NOT move along with the points. So if someone owned only a biennial contract and wanted to deposit points to the next (off) year for additional travel, they would have to pay for a HK credit which costs $159 to make a reservation in that off year. It used to be that housekeeping moved with the points, so you always had a commensurate number of housekeeping credits to make your future year reservations.
Even when you have a buffer, it may become a problem. I own an odd 154k contract along with some annual points. I had 5 HK credits this year, and made one large Wyndham reservation plus an RCI reservation (which doesn't require HK). Then I deposited my remaining points (144k) to 2022. In 2022, I now have 378,000 points for use, but only 3 HK credits. But the 4 remaining I have for 2021 will simply expire. It'll probably be fine because I'm more likely to make 2-3 larger reservations in 2022 than more frequent smaller trips. But I'm still leaving HK on the table in 2021 and could end up paying $159 if I want a 4th trip in 2022.