A new playlist can hold a lot of content, and large files take time to come down the wire. The obvious approach would be to wait until everything has arrived and only then start playing. Screenly does the opposite.
A screen plays whatever it already has. If a fresh playlist has five items and only the first has finished downloading, the screen starts showing that one, looping it, rather than sitting blank while it waits. As each remaining file lands, it joins the rotation, until the full playlist is playing as you built it.
The result is that something relevant appears on the screen as quickly as it possibly can. A new deployment does not stare back at you with a blank panel for ten minutes. It starts with what it has and fills in the rest.
This matters most on slow connections and large rollouts, exactly the situations where waiting for a complete download would hurt. The screen is useful from the first file, and only gets more complete from there.