eval-when-compile and require

[Emanuel Berg]

I'm reading on with the intention of preparing
the pack for MELPA and now it says [1]

    If you use many functions and variables
    from a certain file, you can add
    a ‘require’ for that package to avoid
    compilation warnings for them.
    For instance,

              (eval-when-compile
                (require 'foo))

Here I'm not following. Of course the stuff
should be compiled. And if it isn't, the above
enclosement with `eval-when-compile' helps
by... breaking it?

Besides there are no warnings with plain
`require', so what does it all mean?

[1] (info "(elisp) Warning Tips")

-- 
underground experts united
http://user.it.uu.se/~embe8573