Shellcam is an addition to some artillery shells, making them reveal a small area around the impact zone before impact.
The tactical advantages in Shellcam is negligable, because generally, whatever you can see with the shellcam's espionage camera will be obliterated in a few seconds of game-time. In other cases, the line of sight can only go so far to the edge of pointlessness, because it would reveal either a part of a building, a target that you missed, and even a deflected missile. After all, the enemy should be taking some defensive precautions such as shields or missile defense.
One of the few useful functions of a Shellcam is that artillery can be fired into an area where you have no vision. This means that a Shellcam is capable of revealing a small area that may be heavily coveered by SAMs (preventing air scouts) or otherwise unviewable. As such, shellcams can be useful when targeting high value targets (ACUs, Paragons, etc.) that your enemy may be going above-and-beyond to hide and defend with a Strategic Missile, which will not launch unless it has sight of the area. To use this tactic, first order the Artillery to attack a specific target that is very close to your intended launch point. The projectile will take several seconds to hit its target. While waiting on impact, quickly select your Strategic Missile Launcher (a hotkey is recommended for this) and click the Launch button. Now scroll to the area where you fired the shell and wait until the Shellcam reveals the area, then IMMEDIATELY click it. If you are quick enough, this can allow you to nuke an area you could not otherwise see or reveal.
It should also be noted that, due to the way the Aeon T3 Rapid-Fire Artillery Installation splits its munitions, the shellcam radius it reveals is significantly wider. This advantage is negated, however, by the fact that the Salvation is a game-ender, and unlike the other standard T3 artillery, is more than capable of destroying any target without assistance anyway.