The file "imapsync-2.290.zip" is for Windows users.

The file "imapsync-2.290.tgz"  is for Unix and OS X users, it contains the source
code and also the standalones OS X binaries named "imapsync_bin_Darwin_i386" and
"imapsync_bin_Darwin_x86_64".

The binaries "imapsync_bin_Darwin_*" are for OS X users, as a shortcut for a quick upgrade.

The file "imapsync" is the source code perl script, it is there just for a quick 
upgrade on Unix, just download and replace the old one with this one.
Sometimes new Perl modules need to be installed, see
https://imapsync.lamiral.info/S/news.shtml