Items and fields in Sitecore are used for a number of purposes, the most obvious being to hold content which is presented on the channels and structuring the information architecture for the sites. But items can also be used for other purposes such as configuring the business logic in features or structure the content in with an organisational focus to make maintenance easier. The use of items - and thereby the template on which they are based – helps determines the governance model around them.
Fundamentally it is important that the data structures – i.e. templates and fields - needed by modules are owned by the module itself, as this makes it possible to separate the different modules and the implementation and the logic in them. On the other hand, templates in Sitecore are also the integration point of the pages and sites in Sitecore, bringing together content and presentation into pages and the business information architecture.
The template inheritance technique in Sitecore is instrumental in allowing this separation of concerns in modules: To allow page types to derive from the features required and to allow individual renderings to reference items deriving from the features needed by the renderings. A correct template structure allows this separation while still allowing the content structure and management of the content to be designed with the focus of the owning organisation or specific use case, i.e. with focus on organisation structure, security, languages, etc.