Bluetooth is used as a fallback for creating local offline mesh functionality in case of a full internet shutdown. It will also keep working between devices in the same local wlan, even when the internet connection to the wlan router is cut off.

The idea is to get maximum censorship resistance to the degree where you can still get information flow in case of a full government/military shutdown. You can even sync chats over external storage devices like usb drives or sd cards. All using the same level of encryption as usual ofcourse. Tor is just used because the internet is inherently non anonymous and leaks your identity and location without it.

When you create group chats, for example for a big protest movement, then every device that joins the group will act as a entry and exit node meaning you can mesh together many people with varying degrees of internet access. Below the diagram from their website to give you an idea.