That’s because its job is to convey a certain message. Each graphic design should have a concept behind it, a story you want to tell.
When you see an advert on a billboard or in a magazine. That advert will have been painstakingly put together by a graphic designer to tell you something about that company before you’ve even read any of the information on it.
If it’s been badly designed, you’ll notice the bad design. If it’s been well designed, you’ll just notice what the company wants you to notice.
And, crucially, if it’s been well designed you’ll be able to read the information on that advert or poster quickly and clearly.
So, yes, graphic design is important.