Visuals. In turns over- and underrated.
Yes, you CAN make an excellent game with coloured blocks. You can also make a wonderful movie devoid of visual effects, no doubt. But you can't invoke dragons, magic, spaceships without them. Likewise, too much visual spectacle may smother the story or turn a game into a superficial tech-demo, so a balanced approach is advisable for any graphic artist.
Graphics are for the 'masses'. Because everyone of us has a lifetime of experience processing visual information, we understand them, intuitively. Its easiest to convey information and/or emotions by means of 'the look', and
frequently, the first thing noticed and instinctively liked or disliked in any medium is its design.
Graphics are also a treat - the sugar icing on the cake that is a solid game or script.
Graphics, therefore, are a powerful tool.
Don't leave this important aspect of your project to chance. It will pay off to hire a professional.