Bluetooth wouldn't need a dongle on a modern computer. But I'd be surprised that it's wifi and not some other frequency radio communication.

They always seem to come with a dongle just incase though, and sometimes people use the dongle despite having bluetooth, or plug them in and then connect to the computer's built-in bluetooth.

The problem with Bluetooth is that it just doesn't work well when you want to use one keyboard or mouse with multiple computers.

For this reason I prefer the dongles. Want to use your mouse on a different Mac? Just plug the dongle in a different computer.

I don't know how often I ended up at the password prompt with no way to type the password because the keyboard didn't connect, or was connected to a different Mac, and I had no idea how to get it to connect to the Mac I was trying to start.

This is why I like the Logitech MX keys/mouse. Can have multiple dongles and just press a button to choose which one you connect to.

Logitech also has a Bluetooth variant of this where the mouse will have 3 different "personalities" that you can associate with different devices and then switch with a button. As far as the host devices are concerned it's 3 different mice so there's no switching involved.

It works great and it's the best of both worlds. Convenient and no dongles (Bluetooth on my MacBook was very reliable for a mouse).