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Dezaki With a round table, there would be no "main seat" or "host seats" or "seat of honor".
Image for example.
If there is a rectangular table, it could be like the floor of this image. The "main seat." the one sitting on the smaller end, is the "boss", aka, the seat of honor. Sitting there is like saying "I am superior to the rest of you."
Similarly, those sitting closer to the "throne" are of higher position than the ones further away, also as though saying "I am superior to you." Which probably applies even if nobody sits at the throne. Yes, you can piss petty people off just by sitting down in front. Especially the ancient chinese if whatever I have been translating is to be believed.
Now, turn the table horizontally, so that the pointy ends don't face the entrance and the back of the room, again no one is sitting at the seat of honor. What you get are "hosts" sitting on the inner side and "guests" sitting closer to the entrance.
How about large square tables? Thats probably alright but not as practical as round tables because of the pointy edges, limiting the number of seats even more.
Round tables would eliminate who is more superior or even allegiance if its packed enough. Like Shizuko had said, she wanted to speak to them as equals.
If the question is more towards "why speak on equal standing?" it is so everyone can just speak freely, without worrying about their positions. That way, they may feel more comfortable to ask any questions they may have, among other things.