It's scene stuff, probably taken from usenet. They use it there because files are often incompete and you want smaller chunks for parity purposes. There's no real advantage for torrents.

DCC is a Scene focused tracker. The Scene releases stuff in RAR files. The primary reason for this is that lots of content is on usenet which has file size restrictions, so files need to be split up.

So practically the RAR chunks are the original files for scene releases. If you want an exact copy of the release you need those original RAR files. This can also be relevant if you want to cross-seed to other scene focused trackers.

Bittorrent has no restrictive size limits like this (torrents actually don't have a theoretical size limit, but clients will). So the RAR file is unnecessary and many trackers that have less of a scene focus will extract the files before creating the torrent. Also for common content like audio and video they are already compressed so the RAR is just adding (a tiny amount of) overhead.

It is basically personal preference. If you don't like RAR DCC probably isn't the best tracker for you. There are many others that post just the media files. DCC does tag torrents UNRAR to help you find what you are looking for but there is no way to filter to only UNRAR torrents on-site. However some search tools (like Jackett) will let you filter to UNRAR only.