There are also a number of services that need to broadcast their avialability over the network or perform actions before they become usable.
SAMBA -- Needs to scan network for services/system names, advertise availability and whatnot. Depends on configuration, of course.
Avahi -- Needs to advertise avialability over network. Used for MDNS
Firefly Server --- Service used Avahi to advertise avialability for DAAP service, but it needs to be able to scan the */Music directory for the server to check for updates and whatnot.
Mythtv-Backend --- Needs to configure hardware, scheduale recordings, and it advertises avialability over uPNP for clients and for media extenders.
MediaTomb -- Advertises over uPNP and needs to scan storage.
Libvirt -- It depends on configuration... if it's configured to launch a VM on boot-up then it'll need to be started on it's own, if it's not then the socket-connect method would work.
Nowadays we have a lot of Zeroconfig-type stuff available. SMB/Windows print services, Multicast DNS, UPNP, and 'Bonjuer' (Avahi). Avahi can take care of most of it for Linux/OS X networks (all the service needs to do is register with Avahi for the most part), but for Windows each uPNP service advertises on it's own stuff.
So, basically, there is a lots of different services that need to be started up and shutdown based on network availability and other circumstances.