This is exactly the same question as when you wonder why you call in a professional in any given area. Think about it for a second… Why have an architect design your house? Why have a notary draw up a will?

Graphic designers are experts

Graphic designers are experts in communication and cast a critical eye over the project at hand.

Graphic designers are technical experts

They understand the different stages in the design process, from commission to delivery (printed material or other media).

Graphic designers are visionaries

They are familiar with the multitude of signs and codes in our society and find the essence of what is significant in any given context.