Any event that you are serving drinks or food where your guest count supersedes your own capacity to provide the type of hosting you would like, you will need a waitress for hire to fill in the gaps and streamline your event.
If your event is only offering bar service, having cocktail
waitstaff to attend to your guests drinks, refreshing water glasses, handing out beverage napkins, and cleaning up empty glassware left around your venue, is absolutely crucial. Just one
bartender can become a swamped-looking party. Better to have cocktail waitresses who can flow around the room, get guest orders, and not overwhelm your bar service all at once. Or having ready-made drinks to serve on trays to hand out to guests either at the beginning of service or throughout the entire event. This also reduces strain on your bar service area.
Perhaps you have something more complicated, like a sit-down dinner for a wedding or a corporate party or a fashion gala. You are expecting large amounts of guests at different tables. For this you would obviously need several waitresses for hire, depending on your guest count, to be able to provide the quality of service that your size event demands.