For a solid year I didn't leave the house without Toppik in my hair. Dry shampoo, fibres, check the parting, done. So here's the plain answer on where they help.
Do hair fibres work on bald patches?
Not really. Hair fibres are tiny strands that cling to the hair you've still got and make it look denser. That's the whole trick. They need hair to hold onto, so they look convincing on diffuse thinning. On an area that's basically bare skin there's nothing for them to grip, and you can tell.
Where they do look good
If your thinning is spread out, with fine hairs still across the top, fibres can look really natural. Nanogen especially. With fine hairs there to grab, it gives a fuller look, and with a light hand it's very convincing (the hairline is the bit I slow right down on, because heavy application shows there first).
Fibres won't grow anything or treat your scalp either. They sit on top and wash out with your normal shampoo.
Toppik or Nanogen?
On the fibres alone I couldn't split them. Both are keratin and they go on the same way, from a shaker. You'll find both at Boots. I still point beginners at Toppik, purely because its the one I wore every day for that year. The full side-by-side (shades, sizes and all) is in my Nanogen vs Toppik hair fibres post.
If keratin fibres make your scalp itch, swapping between these two won't help, because they're the same base. A cotton one like Boost N Blend is the swap to try.
When fibres aren't enough any more
If most of what you're covering is bare scalp, that's where toppers come in for me, and my hair topper vs wig guide is where I'd start. For every fibre and scalp concealer I've used in one place, Korean ones included, there's my best hair fibres and scalp concealers roundup.
I bought my Toppik and Nanogen myself.