I think the main use case is avoiding non-open source alternatives.

If you want something that actually offers more capability then something like node-red is probably more useful, it allows scripting all kinds of custom behaviour over many different protocols. (although even then it's rare to find a real use for it, besides having fun hacking things together, though they make it easy to build your own UI for stuff, which is neat)

Primarily, the use case is that it gives you some useful automation possibilities that aren't available in (for example) HomeKit.

Here's an example: I have an AC unit which is either on or off. I don't want my AC running all the time, but I want it to turn on automatically if the temperature raises above a certain value. That functionality is not possible with my AC unit, but I can use HASS to automate it.

Here's another: I have an air purifier which has an intelligent 'automatic' mode, which is supposed to change the fan setting if it detects higher levels of particulate matter - and to be fair, it does, but only between one of three presets. In reality, the machine has a 'turbo' mode which can be manually activated and deactivated.

HASS allows me to automatically switch back to auto mode from turbo mode in case the PM2.5 rating goes back down to a good level (e.g.: 2).

One other thing it's great for is taking IoT products (like for example smart plug sockets) which depend on their own apps, and integrating them with HomeKit.

I have an iRobot vacuum. It has some settings, like it can automatically run when I leave the house. It also has a Siri shortcut so I can control it with my voice. But if I leave home early to go to work, I don't want it to run and disturb my wife. With HASS, I can bridge it to HomeKit, which can trigger it only if NOONE is home.

I could also use HASS and my security camera to only run the robot if there's no motion detected for a period of time and no-one is home, and it's after my wife finishes all her work calls at 5 but before bedtime, but haven't felt a need to go to that level of complexity (plus my cats set off the motion detector all the time!).

It also gives me the option to buy cheap alternatives and bring them to HomeKit, rather than depending on specific apps.

Given that I had a Raspberry Pi lying around anyway, it was an absolute no-brainer.

I’m the same—I have several smart lights, but I can control them all through Google Home—but found a use for it this past winter. In my living room I have a Big Ass Fans Haiku fan, and the signal for volume up on my soundbar is the same as speed down on the fan. I don’t know how companies decide on IR remote signals, but I’ve never had this happen before.

It really annoys me when I turn up the volume and my fan turns off, especially in winter when I want it on to stir up the air. So I set up an automation to check the fan setting and if it is off, turn it on.

So far I haven’t really figured out what else to do with it.

I agree, I feel like it's only for niche uses. But if you have a niche requirement it can be very useful - I have issues with my ISP provided modem and they won't replace it. I use HA to locally power cycle the modem's plug from my phone (my router still works so local networking is fine)

I'm using mine in part as a homekit bridge. Some of my smart home stuff doesn't play nice with homekit, but I can connect it up to HASS, and then I'm able to open my garage door from my phone easily.