First off, there's no questionable rubber core that becomes toxic when heated. Beyond that, they are heavier and more dense, so they massage clothing better. They also have a texture that works better to lift, separate and fluff. You can also add fragrance oil to them to add scent to your laundry which some people may prefer over the funky rubber tennis court odor. Not only will they work better than tennis balls on all fronts, they will also last longer, and remain much safer in comparison.