Graphic design helps clarify meaning and ease communication from one person (persons) to another, and it does so in a few ways.
We like to look at pretty things. The prettier something is, the more easily the bridge between the people communicating and those being communicated to can be built. If it's pretty enough, it'll actively grab attention.
A strong aesthetic also eases the visual navigation when we absorb information. Studies have been done to show that people will put up with a more painful process if the process is pretty (the study was done with ATM machines).
When a designer works with information, they ensure the headings (as example) are of an adequate weight (hierarchly) so the end user can navigate through the text easily without having to read every paragraph to see how it ties in with the rest of the text. This is a very very basic example.
Within a piece of design, there can be dozens of elements to be included. A designer works to ensure that the most important is read or seen first, with supplementary information only being naturally found by the eye when the previous elements have already been absorbed.
It also helps in ensuring that a brand is carefully handled, so that it is evident to the audience when two pieces of communication (from publications to advertisements, posters, menus, flyers, whatever) are from the same company. A graphic designer will make sure to ensure a consistent experience for your customers through out all platforms.
Nowadays, graphic design is an incredibly broad sphere for application of artistic talents. In particular, it includes:
identity (logo and branding) design
icons and pictograms
interface graphics and elements
big print items such as posters and billboards
Thus, it’s easily seen that modern designers in this sphere have multiple options to apply their talents. In all the mentioned directions, the basic purpose of graphic design is to serve a particular purpose or even a set of them by means and techniques of art.