Depends. If you sell One player for 20m, then leave a new signing as undisclosed, potential sellers might try charging you up to 20m for the next one you're trying to buy off them. If you announce you paid £12m for your new signing, clubs know you are limited to 8m and can only push you that far.
When your person responsible has been pilloried as not really knowing what he was doing, and you've got a great deal? When you want to defend selling a star player and can show it was for an amount too good to turn down?