Graphic design is an immensely broad field. Look around you right now, and anything you see is "graphically designed" - my clothing, my coffee mug, the curtains on my window, the shampoo bottles in my shower, apps, magazine, websites, a credit card, a greeting card, album covers, brochures and documents, that funny meme, and the list goes on and on.
A graphic designer can add value anywhere there is content (text, images, shapes etc.) that needs to get a message across. You're not necessarily creating the content, but you're organizing it in the way that best fits the objective.
One basic example from my own world: a colleague wrote a technical document describing how the analytics work in one of our products. That was a 90 page document of pretty much non-stop text and technical diagrams. It was really hard to see where one topic stopped and the next started, hard to find back a particular place in the document and so on. The document was making it really hard to reach its goal of transferring the knowledge to the reader.
So I took his document and "designed" it to address those issues - with really simple tools: using Styles in Word for consistent formatting of the text so similar content is displayed in a similar way. Using some line breaks and page breaks to visually break up the content into logical chunks. Clear chapter and paragraph formatting to add more structure to the content. Nothing fancy but it helped the document better achieve its goal.
My added value was not in my drawing skills or my artisticness, but in my ability to analyze the content, recognize the inherent structure and then use the principles of visual information processing to organize the content in a way that's more helpful to the reader.
Graphic design = visual problem solving