Like most AAC apps or devices, it allows those without the power of speech due to Autism, Downs Syndrome or even temporary verbal problems to communicate. Tapping on categorized symbols allow one to build phrases that are spoken. The app contains 450 pre-set symbols and more can be added.
The second purpose of the app is to teach language and the relationship between symbols and words. Many children have a limited vocabulary and too many unfamiliar symbols would be confusing and overwhelming. "You can start out and delete everything and slowly build the vocabulary and specifically design the application to meet the needs of a student. I think that's the main concept here." Expressive helps with "limited expressive language communication skills," so it's appropriate for all levels of communication disorders.
Expressive is meant to be used by speech therapists in conjunction with parents to build a child's vocabulary and communication skills. In its simplest iteration, you would start with a noun. When that has been learned, an instructor can add modifying words and build up from there. Although it may sound simple, seeing a picture of a ball and understanding that it represents all balls may not be obvious for some. There is an edit mode where symbols can be added or deleted and a user mode that doesn't allow changes. Without this, a child could inadvertently delete everything.