I can't imagine using YouTube without this addon anymore. 1.1x speed lets me go through videos a bit quicker without much impact on the experience. I can watch a video while scrolling through comments like on mobile (and while doing so, the video is at 1080p instead of 1440p). The loop feature lets me loop music that otherwise doesn't loop well.
There are some redundant features such as the pop-out player, but you can hide its button and it won't take up space anymore!