This article on Zwift.com is confusing. They should have said that 'ERG" mode means the demand is set, and pedaling slow, or fast, you have to do that demand. ‘SIM’ means it simulates the efforts of riding outside. You hit a hill, the hill controls the demand based on how fast you pedal. Pedal faster, the demand goes up. Back off, and it sort of goes down.
To me, ERG mode makes the most sense because it’s like lifting weights. Grab a 20 pound weight, and it’s 20 pounds, it doesn’t change by how hard, or soft you lift it. I can see where SIM mode would be advantageous…
I usually ride in ERG mode. Having come from a Peloton bike, the first ride I did in ERG mode, I was excited because the Peloton ‘demand’ is controlled by the rider, and some riders were not as proficient as others adjusting their knob. Some had also figured out how to ‘cook’ their bikes.