What's the benefit of building a useless habit instead of a useful one?

Taking out the emotion. Since the genesis of this discussion was "lifting weights", why uselessly lift weights instead of doing useful work like moving sacks of grain from point A to point B?

"If habit is a muscle that can be developed...", then being detached from the simple, useless habit being formed is good practice for being able to apply it to a productive or important situation.

I can see why; doing something useless requires conscious thought / effort. Doing something you want to do / achieve / change is, controversially, difficult because you think you can rely on just you "wanting it", that is, innate motivation. But that's usually not the case, especially not if it's something that causes discomfort at first (like going to the gym) or if it means giving up or cutting back on something pleasurable (mmm, donuts).

But intentionally doing something that you don't do normally, something you don't want to do, something that doesn't give you any kind of dopamine feedback can help you practice forming habits, practice self-discipline, etc. It's an interesting experiment.

The purpose is to focus on the process of building the habit by exercise of will. Having the habit be something that is useless makes the daily repetition an expression of pure will, rather than e.g. a sense of obligation or desire for a certain outcome.

My guess is that you get practice in habit-building that can be applied to useful habits. Sort of like having students do exercises that have solutions in the back of the textbook. It's not the solution that's needed, it's the practice.

Meditational focus.

learn for a fact that you can change