But why multiply though?

We memorize daily, but do you realize that your brain is like a computer. Whatever you put into it, will most probably remain there for a long period of time. You can choose to fill it with unimportant things, or you can fill it with skills that can be a benefit to you your entire life. Like multiplication. You will use this skill for as long as you live. Using and teaching your brain to memorize is probably the most important skill of all.