Well, perhaps you are in a big war where you have only 2 minutes and your ally needs money and you have a lot of money.
By being able to automatically give this player 1000$ every turn until you cancel it would save some time.
Of course you can also just give this person 10,000$ right away so you don't have to check for 10 turns or something. But what if you have like +2500 every turn but not over 5000$ at the moment? The perfect solution would be this, especially with auto-making units turned on so you won't be out of funds and not being able to send money to your friend.