Typically multiple LAN's are used for 'Teaming' or for LAN Parties for the other PC's. If you have a Home Server or Peer-to-Peer you can effectively double the bandwidth -- sort of like 'Bridging.' Your Router must support these features...most consumer lines do not support e.g. 'Teaming' aka Link aggregation.
This is not useful for Internet, but if you're transferring/sharing/databases large amounts of data then it's very useful.