She's giving a treat to her boyfriend and allowing him to participate a bit before the action gets really going. Come that time, she will focus exclusively on her black man and he will be relegated to watching his girl get totally used by him. These sessions are super exciting for the white woman and the black man. The experience for them is heightened with the audience. For her boyfriend, it is exciting seeing his girl have orgasm after orgasm and be totally taken and experiencing so much pleasure. But it does drive home the point that he isn't the one able to do that to her.