Because the difference between the rates of case A and case B may come from the work of coincidence. Your data comes from an experiment or observation that is not exactly repeatable, they are not accurate, there is a fluke in them. If you would measure again, you would get different values. This distribution causes that you can be sure about one case is better than the other, only if the numbers and differences are big. The greater the numbers and the difference level of the ratios, the greater the certainty. Think about it: If you have a red website with 100 visitors and 10 of them buy something, and you have a blue website too with 100 visitors likewise where 12 of them buy, it may be just the effect of randomness, so you cannot say it with complete certainty that the blue website performs better.