Routing protocols see up/down and adjacencies — never latency, jitter, loss or a dead application. IP SLA injects synthetic probes (ICMP echo, UDP jitter, TCP connect, HTTP GET, DNS) every 60 s and measures what real traffic would feel.

ip sla 1
 icmp-echo 10.0.0.1 source-interface Gi0/0
 threshold 100
 frequency 10
ip sla schedule 1 life forever start-time now   ! no schedule = no traffic!
track 1 ip sla 1 reachability

Then wire the track object into what should react: static route (floating backup), HSRP priority decrement, PBR next-hop. Dual-ISP failover is the canonical use: probe out both gates, steer on health, not on interface state. Responder mode enables UDP-jitter one-way measurement.