3235735 - Use of the application field "referralSource" in Recruiting Management

What is the use case or purpose of the standard application field "referralSource" in Recruiting Management?

SAP SuccessFactors Recruiting Management 

"referralSource" is a standalone field available for customers to use as per their requirement, the picklist for the field can be custom based on client requirements, and the field will be reportable automatically without need of making it customer reportable.

There is no system use of this field and it is not required for any functionality, it is simply a picklist field at the disposal of customer requirements.

It appears in the implementation guide as a standard picklist field: 

Job Application Template Standard Fields

2081566 - Source and Candidate Source Columns - Recruiting Management

source, purpose, referral, picklist , KBA , LOD-SF-RCM-APP , Applicants and Job Applications , How To