Why would Welnix do this? How is it different from just paying them?
Because leave is a debt on the balance sheet so it makes the books look better. More practically, if they use their leave now then the Phoenix are free to play the players until the end of their contracts without having to pay the unused holliday pay if the league restarts.
It's the same reason businesses don't like it if you let your leave build up over a couple of years, it's recorded in the accounts as a liability.