This is not a weapon per se but is a very useful tool that does replace your weapon. While using the motion tracker, you won't be able to actively use your weapons but aiming will remove the motion tracker and bring up your weapon. It is a standard tool that you will always have at your disposal.
Unfortunately, the device doesn't seem to be particularly important or useful when fighting off Xenomorphs but it is remarkably useful for figuring out where you are going and finding collectibles such as dog tags.
