The problem with WordPress not being able to install plugins or update itself is because WordPress uses the Apache ‘user’ for all its processes. In the case of Ubuntu / Linux the Apache user and group is www-data:www-data.
By doing sudo chown www-data:www-data /wordpress your changing the ownership of all files.
A problem can occur if your user account is not in the same group as www-data for example you FTP’d a new image to your wordpress folder and it’s permissions were wrong, then it may not show up on your website. So you should also change your users Primary Group to www-data