Typically, built-in webcams are pointed at you, not at your targets.
You might be saying "Well, I can just point the screen/camera back at the targets and shoot and look around to see the screen" but, most likely, you will soon tire of this routine and will also likely experience false hits and lighting advisories when you move between the camera and the target to look at the screen, as you are causing changing light conditions that can often look like laser impacts.
You may also have a camera that is pointing out from the screen, that seems like it would work well... until you actually try it. With the camera fixed to the device, you are very likely to bump it when you go to change settings or do whatever. This bumping will cause your target zones to be askew to your actual targets and your hits to be askew to your actual hit locations, and will also cause false hits and lighting advisories, as LASR sees changing light conditions that it is trying to make sense of.
External webcams are going to give you the best interface and user experience.