"There is sense in making sure children know why they are doing something, not just how to do the method."
This is an utterly tedious straw man argument. Absolutely nobodyon this thread or elsewhereis advocating making children memorize procedures without understanding them. Nobody. Singaporean teachers teach children how multiplication works and how to use it--but they also give them lots and lots of practice.
I really don't understand why advocates of constructivist approaches to maths seem to think that "understanding how multiplication works" and "knowing your tables down pat" is some kind of mutually exclusive binary choice.
As others have said, yes, there is value in making children do lots and lots of problems, even when they seem to know how to do that kind of problem already. Later on they will be tackling algebra, trig and other things where you have got fractions and powers and negative numbers and multiplication all going on at once in the same problem. They need to be absolutely pat on each of those types of basic operation, so that they can devote the "thinking bit" of their brain to focusing on the algebraic or trig. part of the problem.
If you are still having to fumble about in your brain trying to remember how to multiply 10 to the power of 3 by 10 to the power of 5, and going "Oh, is that what you do to divide a positive number by a positive number...? Erm...." "Let's see, what are seven nines again? Hang on, wait while I faff about doing a gimmicky trick on my fingers to help me remember...." then your "working memory" simply has no space left over for focusing on the higher-order part of the problem.
What's more, if you make a mistake, you will have no good way of giving yourself feedback on your errors or working out why you went wrong--"Well, maybe I made a mistake with the multiplication... or perhaps I arsed up the fractions bit like I do sometimes..... I've never been quite confident with those..."
It is very intimidating and discouraging for students. That is why students who have not been really well drilled on operations during the primary stage have a tendency to scrape through during the early secondary school, but then collapse when they start doing things like algebra.