Ko is also called a "noun emphatic". That means that it emphasizes the noun.
Ko maunga tēnei
THIS is the mountain!
Imagine that you are Miss Marple, oe 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.
Or you've been driving around looking for the house that you lived in when you were five. You see a house
He whare tērā
That's a house. He whare tērā. He whare tērā.
But then you see the house, you didn't recognize it previously because they added on a second story
But you exclaim, in adulation
Ko whare tērā
That is the house!
And everyone goes, yeah, whatever. He whare tērā. It's a house. No one cares. They don't understand.
Bastards.