Initially, Promoted Videos were shown on YouTube search results and then on related videos. Now, if an AdSense site has ad slots for 300×250 Medium Rectangle, 336×280 Large Rectangle, 728×90 Leaderboard, 250×250 Square, and 200×200 Small Square, the YouTube Promoted videos can show up there as well.
Yes, AdSense already has video ads, but these are different, in that these are YouTube videos and not video ads published directly to AdWords. To learn more about the standard videos, click here. To learn more about these YouTube Promoted Videos, click here.