Code Snippets is a handy plugin that makes adding custom functions to your site with ease rather than having to edit a functions file. Before discovering this plugin I use to maintain a plugin that simply had all of my custom functions in it which I would copy across to all of the sites that I managed. The problem with this was that it was quite a manual process as I oftentimes had to make edits in the plugin to suit site by site per case items.
Code Snippets allows for you to export and import the functions that you create within it, as well as giving you options to when you would like the functions to run such as only the frontend, the dashboard, or everywhere.
In the end, if you are looking for a simple way to add functions to your site without editing a functions file, or maintaining a plugin with all of the features this might be right up your alley. And as mentioned also very handy for when you are managing a lot of sites and utilize some of the same functions across them all.