Not necessary with /* Only needed if you do rm -rf /

You can install packages from other places and create your own (and then install them). The distro maintainers have one (or multiple) list of “approved” software but you can add as many lists as you want to your package manager. Often software developers will have their own package list that contains only their own software and if you install it you have to add that list to your package managers trusted software locations. In that sense it isn’t really better than going to the developers website and downloading an installer on windows but it is quite rare you have to do that