Butch lesbianism has a long and complex history; primarily, though, it boils down to subverting typical standards of femininity (which are usually designed around being attractive to men) by finding new ways to be attractive that are geared away from getting male attention and towards being attractive to women. It also has links to working class lesbian culture in the mid 20th century but I’m less clued up about that particular aspect.

(Also “femme” lesbianism - ie. being a lesbian who appears to conform to traditional standards of femininity - is subversive in its own way because it’s about performing femininity but with the goal of being attractive to women rather than men.)