Tobe, the African-American servant, is Miss Emily's only connection to the outside world. People see him coming and going from her dilapidated house carrying a market basket. As she refuses to leave the house, he is the only way she can get what she needs from the outside world. He allows her to live in total isolation.
In addition, Tobe serves to maintain a veneer of faded elegance. When visitors, such as the tax collectors, show up at Miss Emily's house, he lets them in, maintaining the appearance that Miss Emily is a still a genteel southern lady of economic means. Miss Emily cannot move on from the past, and Tobe is a sign of her archaic ways. Tobe also ceremoniously ushers visitors, such as the tax collectors, out of the house so that Miss Emily doesn't have to deal with them when she no longer wants to do so. During the entire time he works for Miss Emily, he betrays absolutely nothing of what goes on in Miss Emily's house. Therefore, when she dies, the townspeople did not even know she was sick. He also betrays nothing about what happened to Homer Barron.
Tobe enables Miss Emily's fantasy life; he is the most immediate representative of the town, which serves her without challenging her.
Tobe is also a sign of how far she's let herself go, for he does things that male servants should not, such as keep the kitchen up.
Finally, Tobe is a marker of Miss Emily's position in history. That is to say, their relationship is a residue of the race relations of the South, and would not have existed elsewhere in this form.