I can't attest to whether it helps prevent cavities or gum disease or whatever, given regular dental visits. I can say though that I used to rarely floss, and since I started flossing daily a few years ago, my teeth remain noticeably smoother and cleaner feeling between dentist visits, and require considerably less scraping when I'm there.

I don't know about you but I can tell how important flossing is by the amount of blood that comes out of my mouth the longer I take a break from flossing