If you do series or shows on your channel, playlists are the perfect way to organize them and keep all of the videos from the series together in one place. I know I have binge watched many series in on YouTube using playlists, and I know many others have as well. Thus, if you have any sort of series or show on your channel, you need to make sure they are being organized on your channel using playlists. Also, make sure when you are linking to any of these videos in the series, you get the link from the playlist. Go to the playlist the video is a part of and click on the video from there. Then, send, tweet, Facebook, however you promote your videos the link that is in the address. This link will send people to the playlist, and thus, it keeps them watching!
Category playlists are used the exact same as series except they encompass a category/type of video on your channel. For example, if you make Call of Duty and Minecraft videos, you would have a playlist for each game. Thus, if someone wanted to watch Minecraft videos, they could go to the Minecraft playlist, and if someone just wanted to watch your Call of Duty videos, they could go to the Call of Duty playlist. This allows your viewers to filter the content they watch more easily as some of your viewers may hate Call of Duty and love Minecraft or vise-versa. This allows them to watch whatever game they want with out having to skip through videos searching for what they want.
Shared playlists are playlists that you feature other YouTubers in. In return, they can feature you in one of their playlists. This is the same concept as sub-for-sub or box-for-box, but it is much better. This allows people to actually view your content instead of just subscribing blindly. Sharing playlists is the best way to get people to your channel these days, so hook up with your friends that are YouTubers, and start adding each other to your playlists!