The jfw_sec_systemuser is used for communication between the CSS and the security client when the CAS and CSS were split, I would imagine that this user is still used in the background today for when configurations are made to the security settings in the CAS.
For more information, please see this forum post:
It is just talking about the organization of the old architecture of DCRUM. The jfw_sec_systemuser was used in tools that used the CSS for authentication such as the Portal, RUM Console and BSM (hence the references to BSM).
Even though you do not have BSM in your architecture it would be best to leave the REST API scripts running as they are just adapted to manage any security or permissions changes in your current environment.