The X7 provides access to files and formats that some players—incuding Apple’s—don’t support.
The Fiio X7 is targeted at music enthusiasts who listen to high-resolution audio using file formats not supported by iOS.
While iTunes is popular, you might not use it because you want to listen to high-resolution music files, or files in formats that iTunes doesn’t support. That also means you use a portable music player that isn’t iOS based but is capable of handling a wider range of files, including high-resolution. Audio files of this type are often distributed in FLAC format (though it’s easy to convert FLAC files to Apple Lossless files and use them in iTunes), or even Direct Stream Digital (DSD) files (a format created for Super Audio CDs, and now sold as downloads from some vendors), which offer an even higher resolution than what’s available in more common formats.