The watt meters on most gym spin bikes aren't very accurate, but if you add a heart rate monitor to the mix you can start to see when you are really working hard (which means gaining fitness) vs. just burning a few more calories per hour than taking a walk.
Echo the others who got trainers with power meters at home - I got a Kickr SNAP 2 years ago, ride over the winter using the Zwift application and having a blast - and can essentially do 45 minute spin class workouts and intervals at home if I don['t feel like driving over to the spin class.
I have not put a power meter on my road bike - when I ride outdoors I'm not going to do training rides really. With a heart rate monitor and bike computer (Wahoo Elemnt) I can see how hard I'm working and if I'm improving my times on common rides and segments - but still focus mostly on just enjoying riding outdoors!