Some people's teeth are so messed up that flossing is a puzzle. "Right angle there, diagonal between those two teeth, curve a bit there, go backward there"...
With a little bit of practice you can easily do it as demonstrated there in under a minute.
Eh, flossing is a placebo anyway.
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