This read-write TruthValue, when true, prevents this port from ever being selected as the Root Port for the CIST or any MSTI even if it has the best spanning-tree priority vector, causing it to instead become an Alternate Port once a Root Port is chosen elsewhere. An admin cares because it lets an admin deliberately keep a specific port from becoming the primary path to the root, for example to avoid routing traffic through an untrusted or lower-capacity neighbor even if STP math would otherwise prefer it. For example, an admin who doesn't want traffic ever routed toward the root through a link to a less-trusted branch office would enable this on that port so it can only ever serve as a backup Alternate Port.