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Because you can, I think, send HDMI down Ethernet with a converter, or hook your Chromecast up to it, again with an adaptor, or stream to some kind of raspberry pi Plex client, should you wish to in future. Mainly because it's easy to route and might be a pain to do it in future if you discover you want it. Just a thought though. Any particular model you're looking at? I still haven't bought one but now have all the cabling in place.
hmmm, what's the reason for the ethernet cable?
I don't think the projector i was looking at even has an Ethernet port...
I was going to buy a cheapish (£250) projector for now because I'm not sure how much it'll actually get used, then look at upgrading later if required.