This thread has been interesting discussion about a question that the OP didn't ask. The OP asked, "Why the need to sit together?", but most people seem to be answering the question, "Why the desire to sit together?"

It's obvious why most people would want to sit together, but other than to give care to a seatmate with special needs, it's still not clear to me why they need to -- especially to the point of seat poaching, getting the FA involved, or having such a sense of entitlement that they raise a fuss when they don't get their way.