Because a bookmark is usually immutable. If I bookmark "http://news.ycombinator.com http://news.ycombinator.com has "Hacker News" it will be same URL forever.

Because a bookmark is usually immutable. If I bookmark "http://news.ycombinator.com http://news.ycombinator.com has "Hacker News" it will be same URL forever. In contrast, git branches symlink to a different ref, whenever you commit to it. A git tag could be called bookmark intuitively.