The sommelier chooses, presents and serves the wine. At an event they pair the labels with the courses, check temperatures and service, and can tell guests in a few words what they are drinking. In Italy training usually comes through courses run by trade associations.
You do not always need one. It makes sense when several wines are paired with the menu, there are notable labels or a dinner where the wine is part of the story. At a reception with a single white and a single red, the chefs de rang handle the service. The bartender often works alongside the sommelier, looking after the aperitivo and open bar.
What to ask
Ask whether the sommelier is included or a separate item, how many bottles are planned per guest and whether you can choose the labels. If you bring your own wines, ask about the corkage fee. The roles are described in the guide to staff roles; pairing ideas in the one on Lazio wines for events.
