what's the purpose of the 2nd pass when encoding or recoding? why not just go 1pass and save tim?E

For most video that we do 1 pass encoding is fine. Where two pass becomes more critical is video with fast motion, high detail, lots of scene changes, explosions, etc... and low bit rates. If I am encoding dance recital at 9-6Mbit/sec one pass would probably be fine. At rates of 6-3Mbit/sec two pass would give a better picture during fast motion and high detail sections. And at 3-1.5Mbit/sec I might go to three or four pass to really optimize the picture.
Sometimes the time difference between one pass and two pass is non-existent, so two pass would be the better way.
Hope this helps.