A lot of apps are just API consumers, but apps with any kind of moderately advanced functionality will require some kind of entity model and business rules that

A lot of apps are just API consumers, but apps with any kind of moderately advanced functionality will require some kind of entity model and business rules that need to be captured. Having those things go out of sync between platforms can be a source of bugs or slower development. For shops invested in Java or with a lot of Java engineers, something like this can be be useful so that the entity model and business rules can be encapsulated in a single code base.