RAR needs extra software to open on most computers. TAR preserves Unix file permissions and is expected by many developer tools. RAR is at home in opening downloads that arrive as RAR; TAR is the usual choice for bundling files on Unix systems.
A password-protected archive is refused, since its contents cannot be read without the password. A plain TAR does not compress anything, so expect it to be about as large as the files inside; TAR.GZ and 7z make smaller files. RAR can be read but never written, because the format's licence does not allow it; that is why there is no conversion to RAR.