Why use free math worksheets? Easy. These worksheets can save you a lot of time and money and when you're a busy homeschool mom teaching several children, this means a lot.
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.
There are other sources for worksheets also. You can find many public schools and private schools which will provide free worksheets for you if you buy textbooks from the school.
Point is, whatever it takes to get students actively involved with the reviewing process where they are not bored and effectively reviewing grade level material in order to prepare them for state or quarterly assessments.