I would also like this feature.
As far as I understand it the way it works on DropBox is:
Client 1 uploads files to DropBox server (external to the LAN).
Client 2 connects to the DropBox server, see a file is there, but sees that it is also on Client 1 and grabs it from Client 1 as it is quicker than downloading from the server.
The file being on the server initially I think is to prevent conflicts.
I find this is most useful when you are adding a new client to the system. Rather than downloading GBs of data from a remote server, it grabs them relatively quickly over the LAN from an existing client.
LAN sync (in the way DropBox does it) is not an issue with a locally hosted OwnCloud server.