rozman summed it up. But the boresighter does allow you to check the scope when in hunting camp (after you zeroed and used the device to see where the sights are pointing in relation to the center of the sighter). This keeps you from having to fire the gun to reconfirm zero when it is undesireable to have to shoot....

I prefer an optical boresighter (collimator)vs one with a lazer. The optical collimator sticks up into the line of sight, so you cannot forget to remove it before firing!