Phabricator allows uploading images locally (to the separate domain phab.wmfusercontent.org, to be precise), and it appears that people find it useful to be able to illustrate tasks etc. in this way.
It would be nice to be able to embed images and other media from Commons, too, e.g. for charts, screenshots, mockups and screencasts. According to the Remarkup documentation, it's already technically possible to do that for YouTube (after setting a configuration variable), see also source code.
Besides avoiding extra uploading efforts for one's own files, this would also enable reusing freely licensed media by others from Commons, and enable reusing media from Phabricator elsewhere.
See also T76247