You can do something for years without getting much better at its hardest parts. Familiar routines let you stay within what already works. Deliberate practice interrupts that comfort by isolating a weakness and arranging attempts that reveal whether it is improving.
Focused work on a skill differs from simply accumulating experience. Feedback has to identify something the learner can change, and there must be another opportunity to try.
Make the error small enough to work on
A violinist struggling with a transition might slow down four bars rather than repeatedly perform an entire piece. A community organizer could rehearse explaining a disputed proposal, ask a partner where the explanation became unclear, and revise that passage. These are different domains, but the practical move is similar: make the problem observable and the next attempt manageable.
Other people can supply what self-correction lacks. A coach notices a pattern the learner cannot yet hear; a peer provides the audience reaction the learner cannot generate alone. Vague praise or criticism offers much less to work with.
Practice is one part of achievement
Practice is not an explanation for every difference in achievement. Opportunities, instruction, health, and the demands of the particular activity still matter. Its practical value is narrower and more useful: it changes an ambition to improve into a sequence of attempts from which someone can learn.