In Hall-Heroult process, the following is used for reducing ${\mathrm{Al}}_2{\mathrm{O}}_3$ :-

The process of extracting Al metal by electrolysis of a mixture of purified ${\mathrm{Al}}_2{\mathrm{O}}_3$ and${\mathrm{Na}}_3{\mathrm{AlF}}_6$ is known as Hall and Heroult process.
Graphite is used as anode which is useful in the reduction of alumina oxide to the $\mathrm{Al}$metal. The graphite electrode reacts with the oxygen released at cathode to form carbon dioxide and carbon monoxide.
Therefore, the role of graphite as an anode in the extraction of Al is to check the formation of oxygen. In other words, we can say its role is to prevent the oxidation of $\mathrm{Al}$ back to ${\mathrm{Al}}_2{\mathrm{O}}_3$ by oxygen.