subdirs works for emacs but fails for me -- why?

[rustom]

Can someone explain the subdirs.el mechanism?

A subdirs.el in the load-path typically has the magic mantra

(if (fboundp 'normal-top-level-add-subdirs-to-load-path)
    (normal-top-level-add-subdirs-to-load-path))

When emacs finds such a file it adds the subdirs to the load-path
If I put a similar file in my load-path it does not do any good.

Also why the fboundp stuff? And not just a simple call

(normal-top-level-add-subdirs-to-load-path)