The purpose of a cocktail garnish is to frame the drink: It influences how you perceive and experience it. A garnish hints at both the character of the concoction and the mood of the establishment in which it is served. Think of garnish as clothing for cocktails—a way of changing a drink's outfit.
The framing effect extends beyond the garnish to the rest of the presentation: the glass, the ice—even, when called for, the straw. Indeed, in recent years, many top-tier cocktail bars have taken to serving drinks with reusable metal straws, partly due to environmental concerns about plastic straws (currently under legal attack across the U.S. for their wildly exaggerated contribution to ocean waste) and partly for aesthetic effect.
A metal straw gives a cocktail a touch of class, a sense of solidity and seriousness: Unlike conventional straws, which suggest a cheap and disposable frivolity, a metal straw offers a feeling of permanence and durability, of craft and care. It demands a drink be considered rather than merely consumed.
It also has the unfortunate effect of feeling funny on your teeth and giving the drink a vaguely metallic taste. A metal straw imparts a certain dressed-up sensibility, but as an instrument for consuming booze, it leaves something to be desired.