A DescFile, as the name suggests, is used to describe a software package. The developer will maintain the file in plain text format. If specified in the relman.conf file by the parameter *_descfile, relman will pass it to pkgmake with the proper parameter, so pkgmake can include it in the created SpecFile.
But there is more to be done with this DescFile: A HTML version can be created and transfered to the server hosting the downloads, so it can be used with HistViews download class - provided you enable it in your projects settings (using the *_descscp and/or *_descrsync options). Even package information (taken from the SpecFile template) can be appended here using the descaddpkginfo option in your relman.conf file.