hub is just a generic name for a device that connects other devices, like the thing that connects all the spokes at the center of a wheel
some hubs have special radios or other hardware, some run software to add other features (like accessing away your home when you're away). They'll also have software to let their app connect, which depending on the manufacturer will let you configure addition options that aren't in homekit, or do software updates to your devices.
If you're into homekit, you want a HomePod or appleTV. Those get you remote access to your home, they will run automations when you're phone isn't there, and most of them have a thread radio.
Thread is key for not having to add additional boxes, as it allows devices from other manufacturers to connect.
even with thread, you'll probably still at least need the manufacturer's app for some configuration and updates.