Trees collapse, and typically represent recursive exploration of some particular area, and can act as a kind of task list or reading list. You collapse the tree when you're not actively drilling into that topic.

Combine with a solid session manager (I use Session Manager) to back them up regularly, and they fill in a third space between an open tab and a bookmark: something you only want to visit once, some time in the next few days / weeks, and have no desire to keep around longer than that.