Echoing was was mentioned above. The wheeler is a nice unit and in a former life sold a good amount of them. I have the Sightmark green boresighter that is the OPMOD version, and the Wheeler, both work very well.
They do come in handy if you're sighting in a few rifles at a range session. Also nice for right after you mount a scope you can go outside and get the scope near dialed in right then and there. I'll line up on a tree about 50-75 yards away, make adjustments, done. Ready to go when I sit at a bench and just the last fine tuning needed.