2 Pass rendering is very useful when using VBR at lower bitrates. The first pass is used to determine where there is more action or change in scene as compared to areas where the image remains more static for longer periods of time. The encoder will then use this information during the second pass to allocate more bits where needed during fast action and fewer bits during slower scenes. This can dramatically increase quality in the fast paced scenes.

With only a single pass, the encoder doesn't know what's coming up ahead of it's current position so it can't "steal" extra bits for high motion scenes as well. It has to always assume that there is more motion coming and needs to reserve bits just in case. Since it can't intelligently use the most bits possible for fast action these scenes may suffer more artifacts and blurring than you will see with 2 pass encoding.

2 pass encoding is relatively meaningless with CBR as the bit rate is going to remain constant no matter what. It also is less useful with higher average bitrates as there are plenty of bits available to handle the motion already. You'll see a marked difference with 2 pass encoding at average bitrates around 256Kbps to 2Mbps. Once you get above 3Mbps you probably won't notice much difference. If your VBR settings are something like 8Mbps peak / 6Mbps average then 2 pass is almost entirely a waste of time.