Why are there 2 ethernet connections on the soundbar? Not that I use them.

A few reasons come to mind:

To use a Connect:AMP (and its connect speakers) as surrounds requires the two devices to be connected via ethernet since the AMP does not have the necessary 5Ghz wifi radio to connect as surrounds otherwise.

Allows for wired use as opposed to requiring wifi only.

Some will "daisy chain" the PlayBar so a smart TV, or other device, can connect to the network.

As I understand it, the device will default to wired if possible, and if you don't have any other devices ethernet wired to your router, it allows the Sonos system to utilize a wifi mesh network amongst themselves that routes all internet traffic back through the wired device - this can allow you to off-load the Sonos devices wifi communication off the router's wifi demands and separate the channel it operates on from your router to better manage the wireless environment /noise/interference if you need to.

As for your purposes....if all works perfectly over wifi, then obviously there isn't a need, but you know you have options if you need them...

Actually I tend to prefer wired connections over wireless. Lot less hassles. Plug it in, good to go. I spent almost a year running multiple runs of Cat 6 cable all over my house. When I look at my AV rack, I have internet connection for TV, CD/SACD player, bluray player, music server, DirectTV box, internet tuner, heck even my power conditioner has an internet connection. Surprisingly my AV controller does not have one, but I suspect its coming with next update.

Even though Sonos is very robust wirelessly, I agree that wired always has to be preferred, so if you have that option I would go for it. Note that, as SHARKB8T mentioned, if you don't already have any of your Sonos components wired, then wiring one radically changes the way Sonos operates, switching it from "Standard" to "Boost" mode.