I once had a friend over twice my age who would do nothing but talk. Sometimes I wished he would stop, but now that it's all over I mostly appreciate that I sat and listened. I listened mainly because he was old and knew so many things, some of them even turned out to be true. I felt because I was young and I seemed to be one of the only people listening that I had to listen, that I was to learn all the secrets no one else was patient enough to hear. It did not occur to me at the time that beneath that, I was happy to listen, joyful for the company, pleased that we had a proper and good exchange of respect, and took turns(albeit with the balance tipped in his favor) to listen to one another and work through one another's thoughts.
Beyond survival, the root point of having other people around I think is this gestalt; to talk and enjoy one another; to let each other gently tug on one another's perspectives, widening our conception of the world and thought itself. Digging through concepts half remembered, mashing them like legos into creations sometimes new, and sometimes simply comforting and familiar. Conversation is everything. Now that I am a bit older, though I am still half his age, and will be for a bit longer before he finishes out the last leg of his decline(his mind went first, his body soon to follow, as these things go), I think talking is all I'm here for.
Instead of talking I write. It'll last a bit longer. It reaches so much further than my voice. I don't have to look the reader in the eye the next day when I say something too offensive or tender. But there will still be a mingling of thought-- yours and mine. In the more immediate sense, writing also allows me to have a conversation with myself, both in the immediate act of writing, and in the looking back over old writing, synthesizing it into new form as current thought weaves and grows from the old like a vine strangling a great oak tree.