Flossing is a critical part of keeping your teeth clean. A toothbrush will clean the smooth surfaces of your teeth very well, and will also reach into the crevices under your gumline.
One of the things that can make flossing difficult is if the floss shreds. It will shred if you have rough dental work that involves the contact point, or if the teeth are too close together. For this reason, waxed floss has become popular. The wax helps it glide through the contact point more easily and also strengthens it to resist shredding.