Supports in different lanes are a difficult thing to pull off, but if you do, oh boy there's a reward for your team.
Sona for example, becomes a nuking monstrosity that can wipe an ADC before they can blink with a Q auto.
Nami has a good amount of burst as well as being able to heal and empower autos on allies (when the E's bonus becomes 200-300 magic damage on hit, it becomes quite unpleasant for the enemies.)
Typically supports make excellent use out of non-support items, they simply can't afford them in the support role. This often comes at the price of waveclear, however.
I have tried sona mid, rakan mid, nami mid, nami top, and soraka mid back when the Q had more range. They were all quite comical, especially soraka back in those days. You would land a Q on one enemy and heal 1/2 your hp from the bonus, as well as being able to refill allies quite easily.
I'm quite sad that they're forced down bot lane despite all of this.