The Keyboard Place Switcher mod simply creates a keybind you can toggle your Use Item/Place Block keybind too. You can change the keybind to do this in the keybinds settings, and what keybind it is set to via the Mod Menu.
Why would you use this? If you use keyboard place, you may often not want to always be using it, but can't be bothered to go into settings to change your keybinds all the time. That's why, this mod simplifies that entire process down to the click of a button! Great right?

