That's a good question and if you've tried to teach a youngster their multiplication tables you might appreciate it a bit more. Let's say you want to teach the multiplication table up through 12x12, well if you memorize all possible combinations of the integers 1 through 12 you'll end up with 78 facts to memorize. Ugghh.
Perhaps if we try to reason our way to this we might have to memorize less facts. For example memorize the squares of the first 12 integers (that's 12 facts) and then learn that near squares are whole numbers away from the squares (that's another fact). So, we know that 6X6=36 and since 6X7 is one multiple of 6 more than 36 it must be 42 (or you can go down from 7X7 to 7X6). You can use other shortcuts to cut down the number of facts you have to memorize for the complete 12x12 multiplication table. The essential point is that you teach kids to use reason in addition to their memorized facts. That's the difference.
So why is 6 times 6 36?