There's no master answer to that. But I like to think of 'graphic design' as a means of communicating information visually. Yes, it's a broad one. But I've been a professional designer for over a decade, and there are many overlaps between web design, graphic design, interface design, UX design as we all have moved online.
The key point is that humans are incredible visual processors - our eye transmits data to our brain at about 1.2 megabytes/second. By comparison, the average adult reads at about 300 words/minute, or 5 words a second. So if you had a second to tell someone something really important, would you use words or images?
The internet is a visual system of communication and businesses on the internet must tell people about what they do through an combination of images, graphics, and text - or graphic design! (The overlaps with website design, interface design, and brand design is a topic for another blog).
So when we want to tell other people about what we do - especially people who are in the middle of doing something else - and thus do not have time to learn about what we do, a visual graphic is a fantastically effective way to tell them.
The key to understanding how we 'talk' through the internet:
It's all about time!
And to earn someone's time, you first have to get their attention effectively. This is why graphics and images are so powerful on the web. Remember flashy ads, pop-ups, and moving targets? Those got people's attention, but they were so annoyed it wasn't effective attention. (Instead, they look for the fastest way to make it go away.)
You can capture someone's attention, and tell them something about your business very quickly. Hopefully in a way that they remember.
Why we remember. And why branding works.