Quickly draw anything symmetrically. Many things in the world have a large amount of symmetry. These aids will automatically mirror your strokes across the x or y axis.
When a mirror toggle button is enabled, you are allowed to move the axis around by dragging it. The axis is always perfectly vertical or horizontal. You cannot rotate this tool. If you try to rotate the canvas, the axis lines will rotate along with it. You will need to use the Multibrush Tool to achieve that type of flexibility.
A common usage for the mirror mode are quick concept sketches. After I did each design on the left image, I moved the axis line over a bit. Perfect symmetry can look a bit dull and boring. If the design needs to show personality and character, make sure to go back and add some asymmetrical details to make it more appealing. The mirror mode can put you in this hypnosis where you do a lot of strokes randomly. Try to think of a direction you want to go before you start using it. It can be easy to spend a lot of time scribbling lines without anything coming out of it.