The source tarball contains a prebuilt binary for Microsoft Windows. They are usually provided for the convenience of users. These files usually just take up space in the tarball.
However, they may be a DFSG violation in that the corresponding source and build system are not available.
Check if upstream also provides source-only tarballs that you can use as the upstream distribution instead. If not, you may want to ask upstream to provide source-only tarballs.