Compact, inexpensive, and LASER. Enough said.
We don't appreciate how far technology has come to provide us with a very focused beam of light that is mass-produced and reliable. Amazing stuff, considering you used to be able to get lasers only by flashing intense light at very expensive optical rods. And here we are now, presenting them to you almost for chewing-gum applications ("Let's stick a laser on it!").
Granted, these are not high-output, nor optically exact, but they'll work great as part of a sensor-beam breaker or simple spot indicator. If you're looking for precision and intense optical exactness, these are not what you are looking for.
Although these are coarse lasers, they still should be operated using standard laser-safety operation rules. Check out the Wikipedia page for information (and drop a donation on them. We do from time to time.)