Prior to this, users who wanted to access the comments or suggested videos would have to exit fullscreen mode. Granted that this isn’t that big of a deal, but it is a couple of extra steps that we suppose ultimately could save you some time in the long run. Plus its these types of quality-of-life updates and features that will make using the platform more enjoyable.
That being said, it should be noted that this only applies to videos you watch directly from YouTube’s main website. This means that if you watch an embedded video and fullscreen it, this feature won’t be available (then again you can’t see comments via embedded videos anyway). These changes to YouTube for the desktop should already be live for everyone (if you don’t see it not to worry it should eventually make its way to you), so you can go ahead and take it for a spin if you didn’t already notice it.