The time it takes to go from wanting something to signing into PayPal and buying it might be just enough for you to change your mind. Which is why PayPal just cut out that step, allowing in-app and in-site buys.
If you're in a game, or on a web page, you no longer have to leave to use PayPal to fund your purposes. Instead, using a new tool the company just introduced, you just have to click on the pertinent link. That will trigger a PayPal pop-up, through which you approve the transaction. All without ever having to leave Farmville.