IF there was some reason, first of all, God could give us the ability to understand if he wanted to, as he is not supposed to be limited. Second, it would imply someone is getting something from it, God, us, or otherwise, that for some reason, God can't give in a way that doesn't involve evil. But again, if he is never limited, that shouldn't be the case.

Also, if cancer and other diseases are supposed to exist and kill people for some kind of purpose we don't understand, why do we have the ability to treat, vaccinate and cure those same diseases? If medicine gets to the point of preventing every ailment, then why does that "oh so important" reason for it existing not matter anymore? It would seem if these things NEED to exist, we shouldn't be able to prevent them from happening under any circumstances.