AFAIK, at least for large residential complexes, the reason is that you can borrow against the base rent, mostly regardless of the vacancy rate.
So if you are renting 60% of your units at $4k per month, you can borrow against the income you'd make if you had all 100% rented out. But if you had to lower the rent on the other 40% by $1k to get 90% occupancy -- you'd have a larger income, but be able to borrow less money.
If your goal is to use leverage to borrow as much as you can and cash out before the cycle ends (which I think is what most people are doing in RE) then this is how you do it.
So if you are renting 60% of your units at $4k per month, you can borrow against the income you'd make if you had all 100% rented out. But if you had to lower the rent on the other 40% by $1k to get 90% occupancy -- you'd have a larger income, but be able to borrow less money.
If your goal is to use leverage to borrow as much as you can and cash out before the cycle ends (which I think is what most people are doing in RE) then this is how you do it.