appimaged is an optional daemon that watches locations like ~/bin and ~/Downloads for AppImages and if it detects some, registers them with the system, so that they show up in the menu, have their icons show up, MIME types associated, etc. It also unregisters AppImages again from the system if they are deleted. Optionally you can use a sandbox if you like: If the firejail sandbox is installed, it runs the AppImages with it.