Why do I need to remove old files first when upgrading Drupal or its modules?

Question

I'm in the process of updating my Drupal version 7.41 to 7.43 and started o follow the guide Update procedure (minor version change) where it says

3. Remove all old core files and directories, except for the 'sites' directory, the original install profile in the 'profiles' directory and any custom files you added elsewhere.

and I was wondering why they wanted to have me erasing everything first, can't I just copy all the new content into my ftp folder except the sites/ directory?

Thanks

Answer 1

By deleting all the directories you're ensuring no files are left over that shouldn't be there anymore. A simple overwrite won't always do that.