"When a cryptanalyst starts out trying to analyze a new algorithm, his first thought is probably: "Yikes. What a mess. I'll never make sense of this". So there are all sorts of tricks to help you start to probe into the convoluted innards of the cipher. One of these is to attack a weakened version. Later, he may be able to extend the attack to the full strength version; or, if this cannot be done, the reason why it can't at least gives some insight into the strengths and weaknesses of the cipher."