The reason behind this procedure is to enable you to achieve the maximum gradient on GA possible in the Engine out case. Say you are returning to the airfiled of departure having taken off at max TOW. Pansops minima (unless steted otherwise) require a 2,5% gradient for the miised approach. Lets say that Icing conditions are prevalent, the OAT is 8degrees or less, low pressure (1000hp) now calculate your GA gradient with corrections from your performance manual. This is of course an emergency situation and under those conditions it is not a requirment to check this prior to T/O but it would be prudent prior to committing yoursels to a 1eng approach in minima conditions. In the CATII/III case it is mandatory to satisfy this case at all times. On a 737 of course you can only make these approahces all engines. Have a look at BSL where the CATIII minima is based on 3% GA gradient, If you are flying a -300/-400 you might find that your performance limiting LW is interesting. Have fun!