Why vary resistance? Most athletes learn to handle one resistance setting, the National Team standard being IIRC DF120, though anything between 100 and 120 is fine. Changing it tends to throw people for a loop.
I rowed in college and have countless hours on concept 2 rowers. Outside of very specific drills I picked one weight setting and stuck with it. If I wanted to vary the difficulty I just rowed faster.
I don't think varying the weight is good for your form and it could be dangerous for your back if your form is bad. After all, you can't vary the resistance of water in a real boat (I suppose you could pour gelatin in a lake). Intensity could be varied just by changing the stroke rate.
good question. Aviron's resistance settings are used to enable high intensity interval training (HIIT) and workout variety for the average person/ fitness enthusiast. For example we have strength programs that guide you through bicep curls using the bar (with an animation that shows feet firmly planted on the ground and a low and steady motion, if you're moving too quickly Aviron will sense it and tell you to slow down your movement)
I don't think varying the weight is good for your form and it could be dangerous for your back if your form is bad. After all, you can't vary the resistance of water in a real boat (I suppose you could pour gelatin in a lake). Intensity could be varied just by changing the stroke rate.