ERG mode on a smart trainer takes the quality of your workouts up a level. Since you are not on TR already, here is a quick progression of workout impact from various tools...

1. Basic Trainer, no workout plan - this just your basic sit and spin routine. You may or may not do intervals, you may or may not have an overall plan for daily & weekly workouts. You just put in the time. You get benefit, but it is limited.
2. Basic Trainer, ad-hoc workouts with power - this is the next level up, using virtual power or your own PM along with free workouts from a variety of apps. (I used the Kinetic app for a while, a long time ago.) You get a crapton more benefits than #1, because you are driving to power and if you have common sense about the workouts you choose, you are getting decent longer terrm benefit.
3. Basic Trainer, training plan - This is where you really take it up a level. You have structured plans over months that vary. All workouts are geared toward your FTP with a longer term purpose behind them.
4. Smart Trainer, training plan - This is the same as #3, but with ERG. ERG mode in a smart trainer changes everything. In all of the above workouts, you can easily slip below the targets and get by. But, with ERG, you must do the power in the plan. And, the other benefit is that you can hit the target power at a wide variety of pedal cadences, which further enhances the workout.

There is an order of magnitude difficulty difference between #3 and #4 above. Though they look the same on the surface, the quality of the workout is very high. It is not an essential, because with good discipline, you will benefit greatly from using TrainerRoad with a good basic trainer. But, you want to get the absolute maximum benefit out of every pedal stroke, ERG on a smart trainer is a game changer.