Amber Server keeps performing a lots of DB activities at the back ground like updating the alert_current, alert_notification tables. Similarly some of the View users might be creating new queries like Saved Hive Queries...etc that metadata is stored int the Ambari Database.
Similarly in large organizations where we may have multiple ambari admins and they might be triggering some operations like add service or making some configuration changes when you started taking a backup. Other admin can be performing some Activities to ambari so the operation history and request tables can be updated.
So it is recommended to Stop the ambari server when you want to perform a Ambari DB dump (backup). This way it is safe and can avoid db corruption. Please refer to the standard procedure of taking Ambari DB backup: https://ambari.apache.org/current/installing-hadoop-using-ambari/content/ambari-chap11-1.html