Redraw is used to remove the remains of lines which occasionally are left in the drawing area. For example, if you move a component, parts of the lines can remain in their original position after the component has moved. Sometimes parts of lines are left behind and sometimes not. This depends on your PC's graphics card.
There are two ways of removing leftover lines:
From the Edit menu, choose Redraw. The leftover lines disappear.
Click in the drawing area (but not on a component). The mouse pointer changes into a hand icon. Shake the mouse slightly.