يُرِيدُ بِهَا وَجْهَ اللَّهِ means that a person seeks Allah’s pleasure through the prayer. يُرِيدُ is a present-tense verb, بِهَا refers back to prayer, and وَجْهَ اللَّهِ here expresses seeking Allah’s favor and acceptance.
Imagine two people praying with similar outward movements. One aims to worship Allah sincerely; the other seeks only people’s admiration. Although the acts may look similar, the report ties its special promise to sincere purpose. A sound intention does not excuse incorrect performance of the outward act.