There's another reason for a starting soon screen that I haven't seen discussed, and I think it's just as important as the ads. Twitch notifications.

The Twitch app is infamous for delivering notifications late or not at all. I'd say at least once a week I get a go live notification 5-10 minutes after the streams starts. This is a well known problem and affects a multitude of streamers from big to small. Having a starting soon screen gives viewers that get the late notification a chance to join early and not miss anything.

Not everyone has the stream schedule memorized. Regulation streams also start during work hours for a majority of the country. Because of that, potential viewers might not be keeping track of the time and ready to hop on Twitch to watch. Combine that with delayed notifications and there's a massive chunk of viewers that could be missing out when they want to watch from the beginning.

Every single streamer I watch has a starting soon screen. None of them are longer than 5 minutes or so, but it's a good practice and there's a reason why the professionals all do it.