2.2. Synchronization
If the local website changes, the upload process has to be repeated to keep the production server up to date. Depending on the size and the number of files, this might take a lot of time, so it would be better to just upload the modified files. Most professional FTP clients offer a synchronization function, which automatically compares the local installation with the one on the production server and transfers the files that have changed since the last upload.
Synchronization is a powerful feature and a real time saver, which is just one of the reasons why you should always use a real FTP client instead of a web browser plugin or the Windows Explorer.