Every mitzvah has the Action (including Speech being a "minor action"), and the Intention of the mitzvah. And even when the Intention is obligatory, nevertheless, this is not the "object" of the mitzvah, but a necessary detail to performing the mitzvah.

Not so with Prayer (expressed openly in Sefirat HaOmer), in which the very object of the mitzvah is the intention (a "Service of the Heart")

Each of the 613 Mitzvot has the goal to bring the Infinite Light into the Finite World. However, this is made possible only through Prayer, which is all about bonding with the Master of the Will, creating a new Supernal Will, from which physically new changes happen (the healing of the sick).