Main thing is central configuration management and rollout to all devices , when you have a network of 1000 plus cisco routers/switches globally and someone in management decides lets roll out new passwords thats when you want prime , do it once push it out
Compliance for running checks against configs and best practices for specifically cisco devices , extremely flexible with programing options/templates
PSIRT reporting eol/eos/eox and software vulnerability checks per image type for bugs
ACI APIC-EM support /ACS view/IWAN support/ISE/WAAS
Prime 3.0 and above has become very powerful and flexible and integrates now with other csico systems