There are two reasons why gamers need to lead their shots when firing on the enemy. The unfortunate reason is because gamers must compensate for lag. In this case, the shooter always needs to be aiming just a couple of millimeters ahead of the where the target will be in order to compensate for the delay of client info reaching the server.
The other very entertaining and appropriate reason for leading one's shots, however, is because one is firing slow moving projectiles across vast distances, and the projectiles require time to traverse the area between you and the target. In this case, the "lead" is not millimeters, but meters and 10s of meters. When the ballistics come together and the target is hit, it is a beauty to behold in the same way that one might appreciate the skill of the quarterback tossing the perfect long bomb to his receiver.
In its day, there were a lot of Halo Blood Gulch movies out there where gamers were shooting from one end of the map to the other and hitting warthogs that were nowhere near the aimed location at the time of release.
Yesterday, I came across a video of some Team Fortress 2 gaming and it reminded me of the Halo 1 glory days of gaming where you need to lead your shots not because of lag, but because of ballistics! The video is of "competitive" gameplay so all the weapons are stock (also included in the 360 version), even though the footage is of the PC version. You will probably immediately notice the mouse-control benefits in the quick-look movement, however, everything done there can be done in our 360 gaming (one or two of the maps are different, however.)
One more thing, please notice the rocket/stick bomb jumping and just consider how much gameplay is available there, that we haven't even tried yet. Seriously, this game is vast and deep, and we haven't even scratched the surface.
You can download it in standard of HD. I recommend the HD version. Watch it guys, it is worth your time. :)
(Also available on YouTube in both definitions.)
UPDATE: Just a heads-up, the "lead" shots don't occur until about the 2:00 (and 4:00) minute mark. Stellar use of the scout and soldier demonstrated before then, but the main lead shots occur after 2:00 or so. Cheers!