This quote is from the book C#: The Complete Reference.

the methods of a class typically manipulate and provide access to the data of the class. With this in mind, recall that Main( ) in the preceding examples computed the area-per-person by dividing the total area by the number of occupants. While technically correct, this is not the best way to handle this computation. The calculation of area-per-person is something that is best handled by the Building class, itself. The reason for this conclusion is easy to understand: The area-per-person of a building is dependent upon the values in the area and occupants fields, which are encapsulated by Building. Thus, it is possible for the Building class to perform this calculation on its own. Furthermore, by adding this calculation to Building, you prevent each program that uses Building from having to perform this calculation manually. This prevents the unnecessary duplication of code.
I was hoping someone could explain this a little bit more. I still don't get why it makes any difference whether the act of this method is in main or otherwise outside of main as it's own method. The method still exists in the class right? Main is in the class. I don't understand what difference this makes to the objects.
Thanks in advance for breaking this down further.

I don't get any of what this line is saying:
Furthermore, by adding this calculation to Building, you prevent each program that uses Building from having to perform this calculation manually. This prevents the unnecessary duplication of code.
How would it be doing it manually?