-It's easy to set up and manage different on-call schedules

-It's easy for someone to manage their own alert settings (e.g. I can add a bunch of different ways to be contacted, like "text me, then call me after 5 minutes if I haven't ACK'd")

-We've integrated it with our internal metrics collections systems so it's really easy to say, "page someone when ${complex_condition} happens"

-IIRC it's not particularly expensive and totally better than the overhead of setting up and managing your own alerting systems.