I have one of these mounted on the business end of my telescope. It is a rifle laser sight. It shines an intensely bright thin green line towards whatever is in the crosshairs of the sighter scope, also visible in this image.
Careful alignment was necessary.
It's useful because I can determine where the scope is pointing or slewing without doing the contortions to peer through the sighter scope.
It works really well. Even at cold winter temperatures. Cold is usually a problem for green laser pointers, but I made sure this had good cold weather specs.
I was aligning the scope's tracking to a star in a section of the sky where there were dim objects that I planned to image. The imager was attached to the laptop and SharpCap was running. I pushed the momentary switch to the laser and saw this frame.