You don't pay the money this year , say you have 12 mil left , and you need a lb and another players let's say a cm who also costs 10 mil . You can get the lb with this method and you are left 10 mil for the cm and you can have both players in the same season .
Second reason is because you don't pay the full amount in one year , you split installments which allows you not to make a minus in the account books