via the maillist discussion:

assuming two (or three) similar directory structures, there are bound to be files that exist in one tree but not the other(s). (e.g. sub/xx.cc.orig) By adding a drag/drop feature (that is, drag a file from any directory tree and drop it on to a new comparison tab) one can now accomplish diff'ing files that do not match up via the directory/name structure.

background:
this feature was requested in 416387, however, at the time, either the "new comparison tab" button didn't exist, or the idea to drag/drop files from the tree onto a blank tab wasn't thought of, so an acceptable UI prevented the feature from moving forward. With the "new" idea of drag/dropping files from the directory listing onto the tab, it becomes feasible feature.