If you want to say this is a book, you can't just say
Te pukapuka tēnei
You have to start the sentence with either he or ko
He pukapuka tēnei
This is a book
Or
Ko te pukapuka tēnei
This is the book
Similarly if you want to say
This is my book
You can't just say tāku pukapuka tēnei
That's incomplete
You could say
He pukapuka tāku
I have a book
Or
He tāku pukapuka tenei
This is my book
Or
Ko tāku pukapuka tenei
This is MY book
"I think this is Rangi's book"
No! Ko tāku pukapuka tēnei
This is MY book
Or, for a slightly different emphasis
Ko tēnei tāku pukapuka
THIS one is my book
"Which one of these books is mine? Not that one! Ah, ko tēnei tāku pukapuka"
ThIs is my book.
So ko doesn't just mean "is". It puts a special emphasis on a thing.
Imagine that you are Miss Marple, or Detective Poirot, looking for the murder weapon. I'm thinking, Colonel Mustard in the ballroom with the dagger.
So, imagine that you are looking through a pile of knives.
You might say
He naihi tēnei
This is a knife.
He naihi tēnei
In the category of knife this is
He naihi tēnei
But then, you come across the one you were looking for... the murder weapon. With blood stains. And fingerprints incriminating Colonel Mustard
Then you would say
Ko naihi tēnei
THIS is the knife!
Noun emphatic.
Emphasises the knife.
I knew it!
Colonel Mustard in the ballroom with the naihi.