Summary
BGP link state is an extension of BGP that allows routers to advertise IGP link-state and traffic-engineering (TE) information to external systems, typically SDN controllers, path computation engines (PCE), or network monitoring/orchestration platforms.
Use case
It helps to introduce integration with SDN controller , PCE, or network monitoring platform which use this information from IGP to create maps or traffic engineer paths.
Additional information
FRR information, PR about it : https://github.com/FRRouting/frr/pull/14560