Creating something new is always a leap forward. And leaping forward is not easy - especially if you are working with a group of people - a team, or multiple teams. In case of products with graphical interfaces, this needs getting everyone aligned on how the new product interface would look like. This is where graphical designers come into play. Graphical designers help put the vision of the new product interface on paper (or software). Depending upon your organization, project or stakeholders, the graphical designer might have different levels of autonomy and control of the future vision - varying from defining the vision, providing ideas to just following instructions and laying things out on the grid based on instructions.
In normal day-to-day life, a graphical designer is often called just a "Designer".
So, when you are planning to redesign the look of the home page of your website or that of your mobile app, you know who to go talk to.