I haven't posted here in a while but it's time to jump back into it...
Sometimes naughty girls are sent to the corner. OK, usually.
I consider this to be a tool of discipline because it is very useful in the process. It can be humbling to stand in the corner which helps the lesson to be driven home. It is also a good place for the spankee to think about her behavior and what got her sent there.
I generally use cornertime in the middle of a spanking, as a break and a time to think. It's a good opportunity to allow the bottom to cool down a little and to evaluate how the session is going. I will also use it after a spanking as a time of reflection. I don't believe in using it for long periods of time, a few minutes is plenty I think. You'll read stories where someone is sent to the corner for an hour or more which I think is a bit much. Now if the corner is the only punishment (say from a distance) then longer periods are appropriate.
Cornertime can also be more 'fun' when used in a play spanking or role play scene. Then it's more of rest period between spankings or just part of the playful part of the scene. I'm very interested in the rituals of spanking and punishment and this is a major one I think.