You can write custom automations between all your smart devices. So I can connect Home Assistant to my phone, a Google Home mini, and Google Translate TTS, so whenever I plug in my phone to charge at night while I'm at home, the speaker tells me "Remember to brush your teeth" in an Italian accent. Or whatever specific weird thing you want. It puts a lot more control in your hands.

Not all smart devices are intercompatible with each other, but Home Assistant is agnostic and tries to work with everything. Most people tend to have automations based on things that Alexa or Google Assistant can't handle.

It may be overkill if you only have a few smart lights that Alexa can handle, but once you have a hundred or more different devices... yeah, managing all of that becomes pretty complicated!

Home assistant has plenty of use cases. it is not only controling devices but also a very powerful automation system. A couple of things I use it for:

-start my laundry only when I have enough solar power to power it

-notify me when my laundry is done

-track energy usage of many devices (heaters, washing and dishwashing machines, A/C,etc)

-let me know when to open or close my windows based on inside and outside temperature

-Force my water heater to turn on when I have solar power

-Expose non-homekit devices to homekit