Why is Vacancy considered a cost?

[Tim Yang]

In the rental calculator here, you factor in vacancy as an additional cost in owning a rental.  I have been just automatically putting in percentages just to do the calculations without a second thought but I just realized isn't putting a percentage in vacancy in the calculator double counting your expenses for holding the property?  

I mean the calculator already factors in your HOA, insurance, utilities, and mortgage,etc. Why is there an additional cost by this term vacancy? Vacancy is already factored in when we calculate the fees I just mentioned. Unless there is something I'm missing here or you actually pay someone because your rental is vacant, I just don't understand why we also have to put in numbers for vacancy when they are already accounted for.