Keeping track of the next action in each project
Any active project keeps track of the next action that you have yet to complete, and styles it differently from the other actions—by default, the next action is purple. This is useful for figuring out what you ought to do to keep your projects moving forward.
When you are only interested in next actions, you can hide everything else. Show the view bar by clicking its icon in the toolbar, then choose Next Action from the action filtering pop-up menu: 
For sequential projects, where you need to do one thing before another, the next action is the only thing you can do right now to make progress on the project. For parallel projects, where you can do the actions in any order, the next action is just the first action in the list, and acts as more of a suggestion of what you might want to do next.