Why do we need to enqueue styles on a hook, why not just enqueue them straight in like this?
wp_enqueue_style( $handle, $src);
the codex page gives an example of using a hook but doesn't explain why (or if) it's important.
There is code that needs to run, thing that need to be setup, before you enqueue. If you enqueue too early those prerequisites will not be filled and you will see ...
... if you have debugging enabled.
The register/enqueue system is complex-- look at the files named
That is not to say that enqueueing a file immediately won't work. It might. It depends on where you try to enqueue. As per the message, you need to enqueue after