Protect the mods finish from the atty and offers a bit of protection from condensation/ leaking. Also useful for getting the af positioned in a certain direction.
I use the 0.5mm ones on just about every mod. Just like Leti said, they protect your mods finish + protects from leaking etc. If you have a nice finish on a mod and pop an atty on without a protector the atty can scratch the paint or dig into the paint. Ask me how I know. Or put an atty swirl on a mods finish.
Good to know. I might grab some on my next FT order.