Another anomaly that occurs when real-time 3D scenes are not anti-aliased is 'pixel popping'. This isn't very clear with a 'static' screen shot so I'll try and explain this phenomenon. Take almost any 3D accelerated racing game (Need For Speed III if you must). If you look at the telegraph poles, for example, they tend to 'pop' in and out. 3dfx showed a demo of Mango Grits' Barrage (shown below) that depicted this effect clearly. The first scene is not anti-aliased. Look closely on the bottom right at the telegraph poles, which show 'broken lines'. What you can't tell is that when this scene is actually 'in motion' these telegraph poles actually begin to 'pop' in and out . The top left show jaggies in the mountains just to re-iterate the point above (as with Quake 2).
Now take a look at the same scene anti-aliased below. The telegraph poles do not have broken lines anymore. Also when the scene turns into real-time and you get multiple frames, the poles do not suffer from pixel popping which you obviously can't tell from a static shot (you'll have to take our word for it).
As Gary Tarolli states in his White Paper, "Rendering a digital image involves a process called sampling, and the technical definition of aliasing is artifacts caused by under-sampling."
To alleviate this problem, rendering techniques often try to increase the sample rate to store more of the original image data. Supersampling is a technique for increasing the sample rate used by 3dfx' T-Buffer.