Tiny tube flies are sometimes used in the UK and Ireland, as well as Iceland, for grilse during hot weather and low water conditions. They are tied on section of tubing about 1/4"-1/2" long and over here are typically fished with a #14, #16 or even #18 treble hook. Patterns are generally just slimmed-down versions of conventional flies - mostly ordinary hairwings, but I have seen mini versions of salmon shrimp flies, too. A similar sort of fly can be achieved by tying directly onto a small treble hook.
I think the supposed advantage over a conventional fly is their lightness and slimness, while still retaining good fish-holding ability. You can see some pictures of tiny flies in the 'Salmon flies' section of this Icelandic site: