when do you need to put nic on the dvuplink in dvswitch?

hi all,

when i was exploring dvswitch before i was always puzzled with dvuplink that you can add nic on it?  so when do you need to put a nic on it?

i just discovered a while ago to modify uplink with lacp vsphere 5.1 and that's the only time i modified a dvuplink, on the web client you can put lacp settings.  is there's some good knowledge base about dvuplink what can it do?

thanks for any comment you may add.