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.

I only tend to dabble, but I have Home Assistant set up - one example I'm on a flexible electricity tariff which is based on wholesale prices. It chages every 30 minutes. I have an automation that grabs tarrif info. If the price goes below zero (which it does sometimes when the grid has more energy than it knows what to do with, my hot water heaters all automatically turn on.

You can definitely have all the gear and not need it. I've set mine up a little at a time to do specific tasks. Some examples:

Alert me if my side gate is unlocked at night, because that is the access to my business.

Check if there is water in the chicken house reservoir, as that means the chickens have dropped a pebble in the valve again.

More cost upfront, but as we've seen time and time against companies will start charging subscriptions for thing they previously didn't.

Then there'd the privacy benefits. Not needing to rely on some company to keep servers alive. Being able to more easily troubleshoot/upgrade/swap individual parts. Not having to use a different app for basically every single device. All that sort of stuff

Others have given examples, but here's one I just did: We have a pellet stove for supplemental heat. Our main system is a heat pump with electric backup - but the heat pump is undersized (thanks, previous owner), so it can't keep up when temperatures get below about 40 degrees (somewhere around there, I forget).

I have an outdoor temperature sensor, and a temperature sensor in the room with the pellet stove. I wired a smart relay controlled by Home Assistant to the pellet stove to act as a thermostat, then defined that indoor temperature sensor and relay together as a thermostat in HA.

Then I wrote this logic:

• If the outside temperature rises above 50, set the pellet stove thermostat to something very low (so it shuts off; the heat pump can handle that just fine).
• If the outside temperature drops below 45, set the pellet stove thermostat to either 70 or 72, depending on whether it's during the night or day.
• Coming soon, once I get around to it: If the HVAC is on auxiliary heat, set the pellet stove to something like 78 degrees, because that will be cheaper than the aux heat.

I'd also like to think about incorporating future info - for example, if the temperature outside is, say, 44 degrees and climbing to above 50 in the next 2 hours, then maybe don't worry about firing up the pellet stove. I may also set something that if it's below, say, 35 degrees outside, then it should keep the pellet stove running no matter what the indoor temperature is.

So here I'm using a Zigbee internal sensor, a 433 mHz outdoor sensor, that smart relay, data from the main HVAC thermostat, and potentially data from forecasts to make my pellet stove operate in a smart, energy efficient manner. HA allows me to take all of this disjoint information and merge it into something useful.

I will likely burn a lot less pellets than I did last year, which also saves me time because I don't have refill the thing as often. The pellet stove will kick on when needed, and shut off when it isn't, and I don't have to worry about it.

Besides, it's kind of fun. ;)