Mistress Nena wanted to humiliate her slave because she did not like him. She disliked him because he was lazy and she did not want him to continue being lazy. So she used her socks to teach him a lesson. As he sniffed her socks which were sweaty and stinky, he learned never to piss her off as she had promised to do worse things to him. He changed immediately.
Princess Serena wanted to test what power she held over this loser. The mistress chose to dominate this loser so she did it with her socks. She asked him to do various things and he did them because they were fun. But she then asked him to lick her feet without forcing him and she found out she held huge sway on him as he did what she asked without question.
Mistress Diana has a fetish for having her socks smelled. And she likes them sniffed when they are smelly or stinky. Today she had walked a lot and her feet were sweaty. Then she got her slave to lick the socks and when he was done, her bare feet. She sat back and enjoyed all that he did for her while he was humiliated and degraded as he did the same thing.