Is "declare" even necessary? What is it for anyway?

I tried leaving out `declare` from my library definitions and it works just the same (I tried `declare type ` and `declare interface`, i.e. I tried removing the `declare` from all those statements and saw no difference).

The documentation is lacking about what it actually does and where to use it. On SO I found someone using `declare` to define something as global but _within his source code_. He then run into occasional somewhat random problems when it wasn't defined - I ventured the guess that if he defines something as global in the source instead of in a lib. definition the order in which source code files are parsed my matter and that global symbols should only be defined in lib. definitions. However, the documentation had nothing to clarify this issue.

But it seems anything in a library definition is global anyway (at least in my short test)? So what is `declare` used for anyway, what does it do?