Your bedroom looks familiar and it's the same room, but it's not in the same place (I mean, in terms of the earth orbiting and all that spinning of stars and wandering of galaxies). And, after you blink, it's not the same time either.
Irreversible forward motion is intrinsic to our universe, but this fact doesn't stop human beings from (tragically) seeking to "keep things the same," or worse, to try to recover their lost "good old days."
(part of a renku that Issa participated in)
"Never turn back" is the first rule of renku. The poets who participate in renku's ritual must constantly push forward, never returning to images or ideas expressed earlier in their chain of verses. This is not just a rule for good poetry; it's a rule for good life.
So this is the first answer to the question, "Why renku?"
Because it teaches us a valuable habit of mind... To look at everything fresh and new because it is fresh and new. To not get stuck clinging to the past (and if we are stuck, to get ourselves unstuck).