Why do you think Chaucer refers to the yeoman as a forester?

Chaucer is simply telling us what the yeoman's job is. In those days, your social status was largely defined by what you did for a living. The forester may not be as high up in the social scale as his master, the Knight, but he has a very responsible job. He's charged with protecting his master's forest from criminals such as poachers and peasants stealing firewood. We can see that he spends a lot of time outdoors from his tanned skin, and the excellent condition of his bow and arrow indicates that he takes a lot of pride in his work. Although the yeoman is a minor character in the General Prologue and is never seen again in the rest of the poem, his presence is nonetheless important, as it highlights the elevated social status of the Knight.