What was important to Plato was formal causality. From experiencing that many existents are (say) beautiful, he realized that there is a form or essence of beauty. And this causes the beauty in things by its presence somehow in them. Formal causality and participation are two sides of the same coin. But Plato came to realize that his view of participation was incomplete. In the Parmenides and Sophist he joined participation with exemplarity and efficiency.
Sweeney, Leo S.J., "Participation in Plato's Dialogues: Phaedo, Parmenides, Sophist, and Timaeus" (1983). The Society for Ancient Greek Philosophy Newsletter. 123.