The drivers that it installs allows for Bridged Mode network, meaning that the same physical network adapter can be used by the VM as well as by the host, as opposed to NAT mode. It also installs a Host Only network adapter that allows the host to communicate with the VMs on a separate network.
If you are not using a VM-only network, you can safely disable the VirtualBox Host-Only Network adapter in Network and Sharing Settings -> Change Adapter Settings
VirtualBox also supports a paravirtualized network adapter, which requires the installation of a driver within the guest, but provides better performance. From Chapter 6 of the manual:
The "Paravirtualized network adapter (virtio-net)" is special. If you
select this, then VirtualBox does not virtualize common networking
hardware (that is supported by common guest operating systems out of
the box). Instead, VirtualBox then expects a special software
interface for virtualized environments to be provided by the guest,
thus avoiding the complexity of emulating networking hardware and
improving network performance. Starting with version 3.1, VirtualBox
provides support for the industry-standard "virtio" networking
drivers, which are part of the open-source KVM project.
