Logos, designs, colors and lettering styles can enhance or detract from the event. They can create moods and images. They can turn on or turn off both potential and actual exhibitors and attendees. Colors and designs that might be appropriate for a banking show would probably seem too conservative for a flower show. Graphics that would be ideal for an auto show might be inappropriate for a medical event.
Graphics can also differentiate a show from competing shows.
Things that stand out from the crowd have a chance of being noticed. This is particularly true when comparing several items. Graphics can be practical. Hanging banners not only add color and flair, but they can direct attendees to specific areas of the show floor. Signage can indicate what's taking place in each conference room or where to pick up the shuttle buses.