What does having two lan ports on a mb do?

PandaButtonFTW

Honorable
Sep 29, 2013
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means you can 'trunk'(I believe that's the term) so you get theoretically twice the bandwidth between your computer and your router.(if your router supports this) it is usually used in servers/switches and isn't of much real use on a consumer grade system.
 

wildfire707

Distinguished
For most people, not much.

For some people (those who go to the trouble of figuring this out), it lets them "team" the network adapters to share bandwidth and keep running even if one adapter fails. It also means that they can connect to two networks at the same time (like the internet and a separate LAN). Computers with two network ports can also act as routers, to monitor and limit access to other computers and network devices.