The files at the archive come in all shapes and sizes. Some are zipped, some are arjed. Some are PDFs and can only be viewed using Acrobat Reader. Some are tarred or gzipped, or both.
(Note: the /incoming directory is not available except through the FTP interface. It is also write-only, so you won't be using it unless you're uploading files.)
When people upload files to the archive, they go into the /incoming directory. Files in this directory haven't been processed yet, and can't be downloaded from there. Once they've been initially vetted and before they go into the main archive, they're moved to the /if-archive/unprocessed directory.
Why do you care about these directories if everything that's put in them will eventually be moved to the full archive? Because uploaded files may not be moved for several days, and many fans of text adventures aren't willing to wait that long to play the latest games. When someone makes an announcement about a new game or tool, check the /if-archive/unprocessed directory first unless the announcement tells you otherwise.