I understand one of the main purpose to use property is for validation and formatting. For example, I have a User class shown below. And I want the firstname and lastname are capitalized when they are being set. Why do I need property if I can write the following code to achieve the same formatting result?

```
class User:
    def __init__(self, firstname, lastname):
        self.firstname = firstname
        self.lastname = lastname

    def __setattr__(self, attr, value):
        if attr == 'firstname':
            self.__dict__[attr] = value.capitalize()
        elif attr == 'lastname':
            self.__dict__[attr] = value.capitalize()

```