An infrared thermometer is a handy device to have, but it isn't really essential in the kitchen. Nevertheless, they have their advantages when it's important to know the temperature of your cooking surface.
An infrared (IR) thermometer differs from an instant read or in-food/in-cooker thermometer by how it measures temperature. Unlike the instant read or the in-food/in-cooker that rely on conduction, an IR thermometer does not make physical contact with the surface being measured; instead it senses the amount of infrared radiation emanating from the surface. Consequently, it is not used to measure food temperatures; it is used to determine whether the cooking surface has reached the right temperature. There are a few caveats, though.
The biggest uncertainty when using an IR thermometer is determining the emissivity of the surface being measured. Without going into the physics of black body radiation, suffice it to say that not all surfaces at a given temperature radiate the same amount of infrared energy - they have different emissivities. Dark surfaces like a cast iron pan radiate much more than a polished shiny surface like a stainless steel pan. In order to accurately measure the surface temperature, one must know what the emissivity of the surface is and adjust the IR thermometer to that value. Two factors save the IR thermometer from obscurity: 1) many common surfaces have about the same emissivity, and 2) high accuracy isn't always required in this application. Most manufacturers set the emissivity of their IR thermometers to a default value of about 0.95; some thermometers are user-adjustable, others are not. In any case, they can be used to make an estimate of the cooking surface temperature.
IR thermometers differ from one another in the angle of their cone of sensitivity. It's a lot like looking through a telescope. At a high power, you have a narrow field of vision. At low power, you have a wide angle of vision. An IR thermometer that has a 10:1 cone will measure the infrared radiation in a one foot diameter circle that is ten feet away. The higher the ratio, the narrower the view. You can measure the temperature of a smaller, more selective area with a higher ratio sensor. If you're measuring the surface temp in a frying pan, you can pretty much ignore the ratio, however, because the circle of sensitivity is much smaller than the pan itself.
Most IR thermometers have a red laser beam that aligns with the cone of sensitivity so you have an idea where the device is looking. At the high end of the IR thermometer scale, the laser will project a circle of dots that defines the area being measured, and these will often be high ratio devices as well. Some thermometers display only the current temperature; others will record the minimum, the maximum, the average, the difference, and the instantaneous values while the trigger is pulled. One product, the ThermoWorks Industrial Infrared, can also accept a thermocouple probe. Another ThermoWorks product, the IR Thermapen, has both a conventional probe and an infrared sensor built into one unit.
If recipes included actual cooking surface temperatures instead of using phrases like "heat until the pan is smoking hot" or "until the oil is shimmering," I could see the utility of an IR thermometer increasing. Until then, these thermometers fall into the "nice to have" category of kitchen gadgets.