When you're using the quest book, sometimes it can be tedious. If you're only a beginner this is not much of an issue, as there are usually only a few quests available. However, if you haven't played the game for a while or on the Test Server, there are dozens of quests to sort through to find and complete "Prerequisites" elsewhere in the book, before you can proceed.
The idea is to be able to double-click one of these prerequisite quests, causing the game to jump to it if it's available.