As a novice salmon angler I can only try to explain how I understand it. This might be my exam, let's see what the seasoned anglers think about it.
A tube is more likely to detach itself from the hook once a salmon is hooked, resulting in less damaged, longer lasting flies.
A tube can be regarded as an extended belly of the fly, positioning the hook further away from the head (even on very long/big flies) without necessitating the use of large/long hooks. In fact a tube can be used with relatively small hooks.
A tube can be light or heavy, depending on the chosen material, and thus influence the depth at which the fly is fished.
The hook can be easily changed when damaged, without having to change the tube and vice versa.
A tube can be used with an inner liner tube, so the dressing can be done/finished on the thin end allowing for slimmer flies and smaller heads.
The hook can be fitted directly into a plastic tube end or by means of a soft plastic/silicone sleeve. These come in a variety of colours, allowing the angler to choose and create a tag according to the circumstances.
Tubes can be weighted by adding a cone or similar on the line in front of the tube. These weights don't necessarily need to be incorporated into the fly, resulting in a more versatile fly.
Tubes can be combined, with different heads, wings, bodies and tags tied on separate tubes to match the anglers requirements or the type of water that is fished.
Hope this is of some help (please correct me if I've got it wrong somewhere).
Johan
Edit: whilst I was typing this, the real salmon anglers did the job, nice and swift