"Run as Specific User" restricts the job configuration only to the specified user. Other users cannot configure the job even if the Jenkins access control configuration allows it.

This option disabled that feature, and the job configuration is controlled by the Jenkins access control.

Use this feature carefully, for a malicious user can run unintended commands without authentication by changing configuration of the job. Check the access control configuration, and ensure that the CONFIGURE permission for this job is granted only to proper users.