Not only is 'Flagging tasks' a neat way to boost their priority (as explained in the 'Next Action' topic), Flagging is generally a super-fast, 2-click process for highlighting tasks that you want to pick out, for any reason at all. So that might be 'tasks you know you can pick off quickly - for a real satisfaction boost' or 'Difficult tasks you've been sitting on, which you're in a mood to tackle this morning'.
Setting Your Tasks to 'Flagged' Status
To 'flag' a task, simply right-click on the task in question and, from the right-click menu that appears (shown below) select the 'Flag Task' option. A small flag/bookmark icon will appear in the top-right corner of your task to show it has been flagged, and the task will move up your project task-list according to the sorting priorities you have set, in settings.