Easy way of exporting selected frame (screen) to bitmap would be very helpful, when you need to quickly share a "screenshot" e.g. via email, slack, imessage, etc.
It's also needed for placing screens into collaboration tools like Jira, Trello, Whimsical, Miro,... various internal Wikis, Dropbox Paper, Google Doc, etc. etc. - you need quick bitmaps all the time.
Sketch way of having a thumbnail of currently selected Artboard in the sidebar, and being able to just drag & drop it anywhere makes it super easy and smooth.