Why not have one? It doesn't hurt. It might help. It gives you a chance to tell people who aren't subscribed about the channel and what your topics are.
I believe a channel trailer is a great way to represent a channel when the channel it'self is new and doesn't have much of a gallery to stand on it's own. My first video was a channel trailer, I literally had nothing on my channel to show, so I basically did a quick video introducing myself, showing some of the art I made in the past and the old "if you like art and you like these pieces, subscribe and you might like what else I publish in the future" Still not a bad strategy I think when you have nothing else to show, it gives anyone who arrives a quick idea of what will be there, even if there isn't anything there YET.
That said, it was about 3 years ago when I think Nick Nimmin and some other suggested trying to use a video with your most watch time and views, and using THAT as your channel trailer, which depending on the length I think could be an even better idea. I did do this for a while (I think I still am? I can't fully remember) but if you're like me, you generally pause a channel trailer when you visit a site, and supposedly 10 second is counted as a view... if someone pauses after 10 or so seconds, that's going to really hurt the performance of the video, so thats why it's probably better to go with a video that has amazing watch time already, because that way it will grab attention and distract your viewer into watching, and might well win the subscribe click before they even get a chance to look at the rest of the channel.
I do need to form a new channel trailer because I kinda think having a quick snappy 1 minute trailer might still be a better strategy to prevent harming watch time of another video that you use as a channel trailer. Whatever you use, you want it to be so good that it distracts the viewer from pausing.
I've also noticed some channels I HEAR the trailer before I actually see it because I don't pay attention to channel trailers normally. They autoplay so if the audio is attention grabbing without being annoying, it could well cause someone to scroll up to it to watch.
