I think it's endurance too. I also think it's surprising that Gin would create step to help knees. I find I have to limit it to avoid tweaking my knees. I would think repetitive movements up and down on the step would hurt rather than help by leading to overuse issues?
Step works different muscles than traditional weight work I believe. There isn't anything I can think of offhand in traditional weight exercises that simulates what is done in step, but please, someone feel free to correct me if I'm mistaken?
I have "rediscovered" step after quite a few years of not doing it and I've followed back in love. I don't care if it's "cool" or not. I don't think I'm particularly "cool" either and I'm too old to care about that either.