It tells you the surface temperature of whatever you point it at, and has a convenient laser for aiming. When cooking, I use it to see if a pain is hot enough yet (e.g. to sear meat), rather than relying on the "smoke point of various oils" test. You can also use it to see how close water is to a boil, although I'm not sure if it is measuring surface temperature, some slight penetration into the water, or the pan bottom (although, arguably, these should be fairly close in water).
It's also useful in something like a fusebox to find hot/overloaded circuits. It's essentially a 1x1 pixel themal imager, while a 100x100 thermal imager costs much more.
You still want a probe thermometer (for measuring meat internal temperature) such as http://amzn.com/B0000CF5MT, and if you have kids/sick people/etc., probably the internal-temperature kind (the ear/IR kind are the least gross).
I have a couple of the Amazon cheapies that I use for checking temperatures when brewing -- it's totally awesome to check the temperature of heating mash water from across the room.
The laser also came in pretty handy for entertaining the most recent foster puppy.
Please elaborate.