The mastery approach is necessary because even when there are very few errors on an assignment, the student is either making simple mistakes or has missed something in the learning of a concept. Either way, it is important to have the student re-work the problems until all are correct. If you simply move on (like the schools do) your child will develop gaps in learning that get bigger and bigger over time. This approach is essential for mastering STEM subjects like mathematics, chemistry and physics. It is also helpful to use in spelling, grammar, writing. I teach to mastery with all assignments at our house. In the following video Sal Khan of Khan Academy discusses the importance of teaching to mastery as it pertains to STEM subjects such as math and other sciences.
See: "Getting Started With Homeschooling."