As with anything training, consistency and structure is what makes us faster, zwift is just a tool to enable that. So I primarily use trainerroad but a few years ago when I was still on a fluid trainer I’d do zwift races. I’m not sure if it made me faster but it definitely helped me with doing threshold for over 60mins. I thought getting a cycleops hammer would get me to have even more fun zwifting but then I discovered the joys of erg mode and the workout library of trainerroad and now I use zwift as a visual while tr does the controlling. I do about 10hrs a week on the trainer and I just derive a lot of motivation from indoor workouts.