Mistress Natasha was angry at her boyfriend because of what he had done and she had to punish him. He had to be punished in a way that he would not forget and in a way that he would always remember. That is why she chose to use her golden shower videos to do it. He was turned into a pee slave and made to drink her pee using a device she bought online.
Mistress Natasha did not want this guy to misunderstand her. He was her slave and he had been doing a lot of things without her permission and she did not want that to continue. So she punished him and she did this with her pee. It was humiliating as well as degrading for him. And he had no choice but to drink her pee in his own bedroom then clean up after her.