Why? Results. Children who master multiplication become adults who understand arithmetic. Arithmetic is far more than multiplication tables, but the ability to perform long division on paper is a sign that arithmetic’s basic issues are welded into your subconscious, and the crucial welding torch happens to coincide with how simple a long division looks to your child. Those tables, once they are embedded in the developing brain, make it so. (Long division requires repeated multiplications – those tables are a win-win.)
I used to enjoy watching the times-7’s (for example) climb upward by adding one to the tens while taking three away from the ones. To a ten-year-old, it felt profound. Your child should experience that.
Oh, and Another Thing 
So – for the next ‘seven’ from 42, you subtract 3 from the 2 in the ones column (assume a circular buffer, so…9) and add one to the value in the tens: 4+1 = 5. Hmmmm. Simple rules, Mrs. Goltz, may not always work?? 🙂
And this is why we sibs either get math intuitively or don’t.
Personally, I love the sevenths. … .142857(repeating); .285714(r) … ; .428571 (r) etc.
LikeLike