With the Insert Freehand Line tool (F), also called the Pen tool, you'll be able to draw any line type you wish. This tool is made to simulate the behavior of a real pen, so it is hard to get a pure straight line or nice curve with it. You will use it when the final result doesn't need to be very precise. It can be used if you like to give a hand-made effect. We sometimes use it to annotate: it happens when I have to report or correct things on a layout.