I’m not a sports physio, but my understanding is that variation is important if your goals are to improve and get stronger. You theoretically could do the same workout over and over, but all you would eventually do is plateau and get really good at doing that workout. Example: let’s say you never do anything other than 2x20 sweet spot workouts. You’d get really good at riding for 20 minutes a time at sweet spot, but if you found yourself in a situation where you had to cover an attack and it required you to do a 2 minute full gas effort your body wouldn’t be prepared.
This is why plans are structured the way they are and there are different training phases. Also there is the risk of diminishing returns. A good example of this is doing too long a build period.
The body is always trying to reach a level of homeostasis and it’s important to prevent that from happening by varying our training and also recovering so adaptations occur.
Also, a 2x20 workout at x-watts isn’t the same as 4x10 at the same watts.