A content model describes the data being stored in the repository. The content model is critical. Without it, Alfresco would be little more than a file system. Here is a list of key information that the content model provides Alfresco:
Fundamental data types and how those data types should persist to the database. For example, without a content model, Alfresco wouldn't know the difference between a string and a date.
Higher order data types such as "content" and "folder" as well as custom content types such as "SomeCo Standard Operating Procedure" or "SomeCo Sales Contract".
Out of the box aspects such as "auditable" and "classifiable" as well as SomeCo-specific aspects such as "rateable", "commentable", or "client-related".
Properties (or metadata) specific to each content type.
Constraints placed on properties (such as property values that must match a certain pattern or property values that must come from a specific list of possible values).
Relationships or "associations"...