MQTT is “just” a way of getting things to talk to each other.
I use it for part of my presence detection, for Zigbee, and shortly for Z-Wave too. It’s not the only option out there, but where both things can use it already, it makes it easier to get them talking.
The alternative after all is that everything needs to know how to communicate with everything else if you want them to talk. That scales terribly badly. Imagine it like languages, if everybody speaks a different language, you have to learn the language of everybody you want to talk to. It’s the same here, MQTT is like a language.
So, people will use MQTT because it works. Nothing more complicated than that. You can add MQTT to your Home Assistant setup when you need it, or never use it.