Why does a Mac Pro have TWO Ethernet ports?

Not one darn thing trivial about asking an honest question... And now that you've learned, so have others.
Glad to have you here. 🙂

This may sound trivial, but why does a Mac Pro have TWO Ethernet ports? I don't understand the rationalization for this. What do you typically do with the second one?

The second port could be used for a high-speed (1 Gbps) link between computers and file servers while the first is used for internet. Internet modems don't have 1 GBPS ports, so sharing one port on the computer would reduce the bandwidth available for file transfers.

Also, I note that sometimes the two green dots on the control panel inside system preferences are lit, when only one Ethernet cable is plugged into port 1 as if both ports were connected to something. Why is this?

If there is nothing connected, the dot should be red.

In my difficult experience of configuring ports and other issues with sharing and so on, it's the Mac and not anything else that is causing 98% of the problem. I've been finding odd things in a variety of programs that only work if I make changes and restart my Mac again.

What the problem is I don't know. Perhaps router issues also? But it's in your Mac somewhere. That's just how I would look at it and then start uninstalling and reinstalling any software involved.

I wish you luck with what can be a really bothersome problem. I hope others more familiar with the topic will write in too. if they don't, start a new topic on this alone and more people will see it.

You can also set a firewall app that runs between the two ports. internet <-> computer <-> internal network

If there is nothing connected, the dot should be red.

But as he sees two green lights, two things are up and connected. Maybe ethernet and something else, like firewire, or modem, or... something else, I forget.

I was actually on the phone with Applecare about this very issue last week. My Mac Pro and Linkstation network drive are both 10/100/1000, but my older Netgear router is limited to 10/100. I thought I'd speed things up by connecting my network drive into Ethernet 2. It *will not* work. I've tried creating different locations with one port turned off, manually entering the IP that came with the Linkstation, etc. Either I can see the Internet on 1, or the Linkstation on 2, but I have to unplug one to see the other. The supervisor at Apple was stumped; he was intrigued enough, he said, that he was going to keep researching the issue and call me back if and when he found a solution.

I was using an older Netgear also, once I upgraded to 10/100/1000 router/switch with latest firmware things are fine.