[lmm]

Because they mean very different things. E.g. "no-one owns this land" is very different from "I don't know who owns this land". "You don't need to do anything" is very different from "I don't know what you need to do". "No-one's in that room" is very different from "I don't know who's in that room". Of course the practical difference depends on context, but so does the practical difference between 2 and 3.

[amw-zero]

That’s not what I asked. I know what the distinction is. I asked why would I ever need that distinction in a program.

In 10 years I’ve never come across a situation where I found that distinction useful.

[thenanyu]

Agree. I would never attempt to write flow control that had different cases for handling undefined and null. It feels so extremely brittle