The VirtualBox Guest Additions are designed to be installed inside a virtual machine, after the guest operating system has been installed, and they include drivers and optimizations for better system performance and usability.
By installing the VirtualBox Guest Additions, the virtual machine should have better video support (including hardware-accelerated graphics), shared clipboard, drag and drop support between the guest and host machines, and shared folders, among others.
Debian (in the contrib repository), Ubuntu and Debian/Ubuntu based Linux distribution users can install the VirtualBox Guest Additions from the repositories (installing the
virtualbox-guest-x11, virtualbox-guest-utils and virtualbox-guest-dkms packages), while Fedora 28 and newer ships with a Guest Additions package installed by default. This way of installing the VirtualBox Guest Additions is easier than installing the Guest Additions shipped with VirtualBox, but it may lack some features, like the ability to use the new file manager added in VirtualBox 6.0.For example, in Fedora 29 which comes with the VirtualBox Guest Additions package installed by default, using VirtualBox 6.0 you can resize the machine window, and enable shared clipboard between the guest and host machines, but no shared files related actions work (can't add shared folders, the new VirtualBox 6.0 file manager doesn't work - it completely freezes the virtual machine, and dragging and dropping files between the guest and host doesn't work).