If your company or business is holding a special event, promoting a new product, or has a new message that needs spreading, then large format printing is the perfect way to let the world know about it.
When traditional promotional methods just aren't giving you the amount of 'oomph' that you need in your advertising, is the next step up. banners and displays have over alternative advertising methods is that they can be viewed from far away. This makes your message far more noticeable to potential customers, and also reinforces it within people who focus on it for longer periods of time, such as when waiting at a bus stop or queuing at a cashpoint.
With advancements in modern printing technologies, more large format ads can now be produced faster than ever before, making it very cost effective. Your money also pays for a decent amount of longevity, as large format banners and displays can last indefinitely in conducive, indoor conditions. Even when used outdoors, they can last several years depending on the materials used and the variable circumstances of the environment.
So, if you want to let people know about your business in a large way, but don't want to spend large amounts of money, then large format advertising is the way to go.