In reply to joshen:
> Or have i missed something?
You've missed something.
1. In the case of a traverse it allows the belayer to move along the top of the crag, effectively giving the climber a top-rop throughout, rather than a potentially dangerous swing. If you've got a nervous second then it tends to be appreciated.
2. It's a lot easier to haul a second up if you are standing.
3. Your legs can take an awful lot more than the climbers weight.
4. If you've got really long anchor ropes then sitting on the very edge is dodgy, as there's a potential for rope stretch to put you over the edge. Standing a couple of feet back means you can tilt the top half of your body and still see the climber, but also allow some contingency stretch.
5. You can often see the climber better from standing anyway.
6. If anything goes wrong you've got more freedom to move, you've not anchored in one place. Getting up from a sitting position whilst holding a belayers weight is very difficult.
However, in general, I agree with you. Sitting is better if anchors are at ground level, but as long as you are aware it comes with its own limitations then you can make a judgement on a route-by route basis as to which is the lesser of all the various evils. There's no hard and fast rules. In some places sitting down may not work, simply from the topology of the rock - sitting at the edge may be unstable, standing a couple of feet back stable.
e.g. I generally belay from standing if it's Lady Blue seconding, as she weighs a lot less than me, and I can easily hold her weight. Also as I tend to drag her up routes that are hard for her, hauling may be required.
If it's my mate Nick (17st) then I'm generally very well anchored indeed and sitting down, and may well have him on a more-or-less direct belay, as indirect tends to be uncomfortable. Saying that, on a recent traverse I was standing, as I judged that being able to give him a top-rope and hence no swing on a route that was top end for him and where he wasn't very confident where he might fall off (he did) was more important than discomfort for me. I held him fine, and certainly didn't buckle to me knees, or even close.
Like I re-iterate, depends on the situation. In most things in climbing there's no totally hard and fast rules, but you adapt what you know to the situation at hand.