I do my workouts without ERG mode which I don't get along with, but otherwise, whether it's for doing a hour and a bit long Z2 spin or a workout session, a trainer paired with something like Zwift is brilliant and translates to on road fitness very well.
Of course if you're preparing for something long then a long ride per week is going to be necessary and that's more fun to do outside, but for building fitness, the turbo is more time efficient than outside, really. You don't waste time - from a training perspective, anyway - descending or softpedaling in shelter or riding to a place where you can do intervals.