When children advance to Grade 3 and they have to add above 100, they can make all kinds of mistakes. This is due to a lack of number sense. Visual cards help children see the numbers and learn why an answer is what it is.
Kids need to visualize 90 + 30. 30 gives 10 to the 90 to make 100. Then there are 20 leftover. 100 and 20 make 120. A common mistake for 90 + 30 that I see is 102,121 or 122. When kids can see the numbers visually, they realize that there are no units to add.