I have fine, pin straight hair and tons of it, so I used to do layers to thin out my hair because it's distributed very thickly across my scalp (it's never actually looked like fine hair because there's just so much of it). Now I'm growing it out and I'm realising just how much hair is actually on my head ...
I think if your hair is straight, layers tend to thin it out, but if it's wavy or curly, layers tend to add volume. So if you have fine, straight hair that already looks thin, layers might not be the way to go.