The circle to land reason other than the obvious is that the weather ie. wind may favour a different runway to the one the ILS/NDB etc approach is on. Hence a low level bad weather circuit to another more into wind runway. On my IR we finished with a single engine ILS to minima then down came the creens and I went around into low level circuit to land on the same runway. In reality with a failed engine and a nice long runway ahead it would be more sensible to land if at all practicle rather than do another circuit. One interesting point is that after 2 hours or so on instruments with screens up the visual circuit can have a sort of "rabbit in a headlight" kind of feel. I have heard of people failing right at the end of the test due to poor visual scan.