What is the role the soul plays for knowledge in Platonism?
Plato notes that the soul has three functions: it is used for desire (of material things perceived by the senses), emotion (used for social interaction), and reason (which is aimed at seeking truth). When a person is able to perceive an object, truth, or thing in its ideal Form, that person's soul (or metaphysical/spiritual intellect) is able to apprehend that object, truth, or thing in its abstract conception.
Plato believed that there are physical things in the world (such as a chair) but there are also ideal Forms of those things. Therefore, the ideal Form of the chair would be the abstract Form; that abstract concept is the idea of the chair's perfect function. Since the soul is immortal and abstract (metaphysical), when a person is able to conceive of something in its ideal form, that person's soul apprehends (learns, understands, or intuits) that ideal Form. Note the connection between the ideal Form of the chair, an abstract and metaphysical concept, and the idea of the soul, likewise an abstract and metaphysical concept; therefore, it is the immortal, metaphysical soul that can grasp such a metaphysical and abstract ideal Form.
Plato also wrote about anamnesis which literally means to remember what you have forgotten. So, there are some things which we might remember from past lives or from the time just prior to being conceived and/or born. The idea is that before we are born, our (immortal) souls exist and are therefore in tune to ideal Forms and certain a priori concepts, but upon being born, we are born into physical bodies and tricked by our senses into looking primarily at the materiality/physicality of things. Later in life, through philosophical introspection and/or anamnesis, we might remember these concepts. In Phaedo, Celebs is persuaded by Socrates that one can learn (recollect) things which we learned or knew prior to being born into human (physical) form:
Your favorite doctrine, Socrates, that knowledge is simply recollection, if true, also necessarily implies a previous time in which we learned that which we now recollect. But this would be impossible unless our soul was in some place before existing in the human form; here, then, is another argument of the soul's immortality.
In any case, the role that the soul performs in terms of knowledge is mostly in terms of apprehending ideal Forms and/or concepts which are only apprehended, learned, or recollected via reason (rather than through the physical senses).