Why does byte compilation affect existing Emacs session?

[Alex Branham]

Hello,

I'm trying to understand the following behavior. I've set the f1 key to
a keymap. Then I open a file with the contents

#+BEGIN_SRC emacs-lisp
(use-package help
  :defer t)
#+END_SRC

and byte compile it with M-x byte-compile-file. Now the f1 key is back
to the help keymap (same as C-h), and not whatever I had set it to
before.

Why does byte compilation affect an *existing* Emacs session if the elc
file isn't loaded?

(Yes, I know I can add :no-require t to prevent this, I just don't
understand what's going on in the first place)

Thanks in advance,
Alex