It all depends on aircraft single engine climb performance. During cat2/3 procedures, you have to assume a go-around from DA. Obviously, you are lower down and therefore have to achieve a better climb gradient. There is a standard CAA and JAA limit but I can't remember what it is. Think it's 2.1% but that might be Cat 1.
In laymens terms, aircraft don't climb as well with landing flap as with an intermediate flap setting ergo better performance if you have to go-around from low down.
