Sometimes it's not possible to use bridged adapters for you VM guests and you need to use NAT.
To access your VM guest from the VM host becomes difficult but not impossible with host-only networking.
As a developer I have a web, version control and database server running on my VM guest and don't want to use port forwarding, example:
VBoxManage setextradata MyLinux "VBoxInternal/Devices/pcnet/0/LUN#0/Config/ssh/HostPort" 2222
VBoxManage setextradata MyLinux "VBoxInternal/Devices/pcnet/0/LUN#0/Config/ssh/GuestPort" 22
VBoxManage setextradata MyLinux "VBoxInternal/Devices/pcnet/0/LUN#0/Config/ssh/Protocol" TCP
See also this article: http://allisterx.blogspot.nl/2008/05/additions-and-ssh-access-to-virtualbox.html for more explanation how to port forward.
