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.
From a teacher's perspective our competition is tough. Passing out a handout of 30 problems that are all in a format of 534x25= is not as stimulating in the students' eyes as playing games such as Grand Theft Auto and Resident Evil.
They're also available for nearly all grade levels. There are printable middle school, high school, elementary school, and even pre-school worksheets.
How much of an apple pie has been eaten? The answer to this question can be expressed in percentages, 50%; or in decimals, 0.5; or in fraction, ½. In other words, half of mom's delicious apple pie is gone.