>>279903181
The imposters not only have narrative significance to the two arc in Nise, but to the series as a whole. The way it applies to the sisters and those stories are pretty obvious. Sisters pretend to be a hero team, Araragi pretends to be human, Kaiki unleashed fake oddities, Tsukihi is not a normal sister, they stop pretending to not feel anything sexual towards each other, Hitagi drops her yandere act, Shinobu poses as a child for the first time, Yotsugi is a character wrapped in unclarity, Araragi and Hachikuji become even more true to each other, Kaiki is a man of many masks, Kagenui claims to hunt immortal oddities while walking around with one and later letting two others go, Nadeko is very obviously putting on an act, Kanbaru gets a bit more real with Araragi, and so on and so forth. As for the series as a whole, I covered some of it in a way with the list, but there is also a much more significant piece to it.

With the way oddities work, imposters could literally become real. As could many other things arise that are a lot stranger yet pretend to be normal. You could pass a dark alleyway everyday on the way to school. In the Monogatari universe, if you were spooked enough by it, it may cause some horrific creature to emerge from it someday. You may also come across something, such as a dead cat, that actually contains a very powerful oddity.

It is important to remember that Nisio wrote Nise quickly after discovering Monogatari would be animated. Some things you may notice are funnier like how he promptly inserted a loli nude scene and incest, knowing they'd have to be drawn with the importance he gave them. But other than that, it seems like he realized he was truly on the cusp of a big thing. So he laid out the groundwork for it in Nise. The quick writing time could also be seen as reminiscent of how writers work in general. Laying out the critical elements quickly to best capture what you envision. Nise is essentially the making of Monogatari