In other words, they never memorized their multiplication tables! Many times I have seen a student do poorly due to a weakness in basic math facts they should have learned in third grade.
Subtract 1- or 2-digit number from a 2-digit number with/without renaming
Worksheets also, when used properly, provide both the students and parent / tutor immediate feedback as to the child's progress. This means they can be used to point out areas where the student needs further reinforcement.
When a child learns to relate math to everyday questions, he will be great at it from the simplest addition all the way to trigonometry. To convert percentages, decimals and fractions is thus one essential skill.