The idea is to get the user in your head as a real human being not as an abstract category. If you design for "User who needs product X" then you come up with

The idea is to get the user in your head as a real human being not as an abstract category. If you design for "User who needs product X" then you come up with different design than if you think about "Jim wants to buy a gift for his nephew's birthday, he is late, the birthday is tomorrow." You can better come up with ways to support Jim. You feel the importance of a speedy delivery. You can also think about new features like asking this user if he wants a suggestion mail sent to him for next year (minus two weeks), that has present suggestions for kids that are 1 year older. Now I just made that up, and it might be a terrible idea, but the point is, that you could never come up with something like that without a persona.