I really like this feature because it makes it a lot easier to see the connections between different pages and tags. While writing, it can provide spontaneous inspiration; dropping a tag and then seeing its linked content can help prompt further ideas.
It also makes it easier to "peek" at linked content for a tag without having to commit to navigating to an entirely new page. In addition to being slower, navigating to another page also entails a costly mental context switch, and can disrupt concentration. Hover previews enable viewing related content without disrupting context.