This read-only INTEGER, per the description, identifies the router instance an NMS wants to manage in the current SNMP request, and must be attached as the first variable in the PDU to scope every other variable in that request. Admins care because forgetting to place this OID first means the rest of the request won't apply to the intended router instance on a multi-instance device. For example, an admin's script includes this OID with value 2 as the very first varbind in a GET-BULK request to ensure every returned interface counter belongs to router instance 2, not the default instance.