Save Money : It's easy to see how free worksheets can save you money. If you want, you can skip buying math books and just use worksheets that you get for free on the internet. All you need to do is use a "scope and sequence" book that tells you what your child needs to be doing in math by age and grade. This book is essential when you homeschool.
There are several standard exercises which train students to convert percentages, decimals and fractions. Converting percentage to decimals for example is actually as simple as moving the decimal point two places to the left and losing the percent sign "%." Thus 89% is equal to 0.89. Expressed in fraction, that would be 89/100. When you drill kids to do this often enough, they learn to do conversion almost instinctively.
Solve vertical and horizontal computation problems
However for the most part, when a worksheet is needed to help drill down a procedure, standard, or lesson, its effectiveness can and will vary.