As event staff, ushers often need to interact with guests in a variety of situations, and it's important that they project an image of impartiality. Dressing in black and white helps them to do this by creating a classic, understated look that doesn't draw too much attention. Additionally, black and white are colors that are associated with respect and authority, which can be helpful in managing and directing guests.
Finally, black and white are colors that can be easily matched to any event's color scheme, making it easy for ushers to coordinate their outfits with the rest of the décor.