The methods in the Selector classes encapsulate common SOQL queries made by the application, such as selectById, as well as more business-related methods, such as selectByTeam. This helps the developers who consume the Selector classes to identify the correct methods to use for the business requirement and avoids replication of SOQL queries throughout the application.
Each method also has some standard characteristics, such as the SObject fields selected by the queries executed, regardless of the method called. The overall aim is to allow the caller to focus on the record data returned and not how it was read from the database.
