I think you should already be clear that the file
functions.php of your active theme is the Pandora's box of any WordPress installation .
By including small codes in this file you can add functionalities to WordPress without having to modify files of the installation so – by the way – you avoid the installation of plugins for simple functions.
Now, once you encourage adding functions can get a little uncontrollable this file, but everything is fixed …
From a simple interface you can add new codes, edit existing functions, activate, deactivate and even delete existing functions all as if it were the plugins screen, in this case yours. In fact, you can even add functions without activating them, because they will only be executed if you "activate them", just like with the plugins.
Another great virtue is that if you change the subject you will not lose the functions , because the " snippets " are saved in the database and will be available for the new theme if you wish.