Laser light is useful in entertainment because it is pleasing to the eye, and because the coherent nature of laser light allows the use of optical scanning to draw patterns or images on walls, ceilings or other surfaces including theatrical smoke or fog.
Scanning
Laser scanners consist of small mirrors which are mounted on galvanometers to which a control voltage is applied. The beam is reflected a certain amount which correlates to the amount of voltage applied to the galvanometer scanner. Two galvanometer scanners can enable X-Y control voltages to aim the beam to any point on a square or rectangular raster. This enables the laser lighting designer to create of patterns such as Lissajous figures (such as are often displayed on oscilloscopes); other methods of creating images through the use of galvanometer scanners and X-Y control voltages can generate letters, shapes, or even images. (The use of X-Y raster scanning to create images is also used in television picture tubes.) A planar or conical moving beam aimed at smoke or fog can display a plane or cone of light known as a "laser tunnel" effect.
Safety
Lasers can cause eye damage if aimed directly into the eye, or if someone were to stare directly into a stationary laser beam. Some high-power lasers used in entertainment applications can also cause burns or skin damage if a sufficient amount of energy (typically a stationary beam) is directed onto the human body. The use of lasers in entertainment, like other laser products, is regulated by the U.S. Bureau of Radiological Health and by some state regulatory agencies such as New York State which requires licensure of some laser operators. Safety precautions used by laser lighting professionals include required interlocking mechanisms so that the beam is always projected above the heads of the audience; or so that the beam moves at a speed sufficient that no harmful amount of laser energy is ever directed at any individual audience member.
External Links: [1] U.S. Bureau of Radiological Health