YouTube music, higher quality video, and the ‘jump ahead’ feature to skip portions of a video that others usually skip.

You can do this manually on the free plan. Mouse over the playhead and you'll see a graph. You can click the highest point in the graph which is where most people clicked through to.

Also, though not a benefit to you in particular, apparently any creator you watch with Premium gets a way bigger payout for your view. Only heard about this anecdotally but it seems to track.