I am a Registered Play Therapist (RPT) as well as the current secretary of the Association for Play Therapy (APT) local Orange County chapter.
From fetal development to the first few years of life, our brains are wired to intentionally and specifically favor nonverbal, sensoriaffective, relational experiences. An example of such an experience is a caregiver reaching to hold and rock his/her infant in reaction to the baby crying. The baby in turn, may cease crying and slowly fall asleep given that his/her distress has been alleviated by 1) the caregiver's affective (emotional) recognition and awareness that crying indicates distress as well as the need for relational comforting and 2) the caregiver's use of nonverbal, gentle, sensory tactics to soothe the child. When an interaction like this is effectively repeated and paired with instances of upregulated relational exchanges (like mutually shared and amplified joy or elation), the infant begins to internalize elements of this co-created, dynamic relationship. Over time, these interactions serve as a neural template for self-regulatory processes and what is known as the child's "internal working model," which refers to the conceptual framework a child utilizes to understand the world, his/her self, and others. Foundational self-regulatory patterns as well as this initial model persist into adulthood; at the end of the day--as adults--we are a collection of all of our life experiences from the time of conception onward.
So how is this information related to play therapy? Well, to put it simply, play involves a series of nonverbal, sensoriaffective, relational experiences that contribute to the development of an individual's self-regulatory patterns and internal working model. In play therapy (a term which refers to exactly what one would expect: the use of varying play-based interventions aimed at psychotherapeutic gains), the intent is to open a channel to the neural networks so highly favored at the onset of life. The implicit (meaning automatic and outside of one's immediate awareness) and symbolic nature of play grants access to a rich surplus of intra- and interpersonal knowledge that is otherwise, inaccessible via traditional "talk therapy." ("Talk therapy" simply refers to therapy that solely utilizes verbal discussions.) The singular use of talk therapy can either be 1) developmentally-inappropriate (for instance, we all know that telling a toddler, "I understand you're upset but crying is just getting you further worked up" will not resolve anything) or 2) if developmentally-appropriate, may at times, overemphasize intellectualization at the cost of nonverbal, sensory, or affect-based needs. Play therapy thus, pays homage to the oldest, most deep-seated, and longest developing areas of our brains. As such, play therapy inherently promotes psychotherapeutic change given it's neurodevelopmentally-sensitive nature.