I think the point is that if you aren't mindful and deliberate with your actions, you'll descend into behaving according to habits and desires instead of what is in the best interest of your future self.

Without being mindful, you'll just have another handful of chips instead of remembering you're trying to be not fat.

Without being mindful, you'll watch another Youtube video instead of doing something on your todo list.

Without being mindful, you'll jerk off instead of reading a book or interacting with another human being.

It's not that doing any of the above is inherently bad in isolation, it's that if you aren't mindful in the aggregate and just default to impulse, you'll find yourself drifting much farther from what you think you will eventually become than you otherwise realize.

We live in a world where distraction and dopamine hits are so accessible (sometimes even out of our control) vs even just 50 years ago, so we find ourselves needing to be more deliberate in our actions.