The database stores the data, the rest API handles the business logic. Django has multiple plugins for providing a REST API, such as Tastypie. <a href="http:&#x2F;&#x2F;tastypieapi.org&#x2F;" rel="nofollow">http:&#x2F;&#x2F;tastypieapi.org&#x2F;</a> With this, django is the API. 
A rest API is good for when you want to share data between multiple clients (eg a website and mobile apps) and want to have more complicated business logic than the database can provide by itself.