Most security systems these days are just whitelabeled zwave etc sensors with a proprietary hub and a monthly charge.

The nice thing about HA is that you can pull almost everything into it and then add whatever automations you want. Recent example was my SO complaining about how dark it was going to the car when they leave in the morning. Super easy to set up an automation that turns on the floodlight switches when the front door opens between dusk and dawn. All kinds of stuff like that that's really useful.