I don't very often load code "live" other than on starting a client in the form of "macros" which are pretty much stuff I don't want to add into the editor core because it e.g. might be short lived or is just too esoteric. Even though my editor is mostly for myself, imagine anything you might put in your .vimrc, except it's only self-discipline that separates this from core editor code.
E.g. I have functions in there to insert headers in my journal for example.
In terms of executing code "actually" live, it's ~80% for debugging when I work on the editor itself. Pry provides all of the plumbing so all the code needed to add that is just to suspend the editors own input handling and call pry, coupled with an exception handler that calls pry as well, so there was no reason not to, basically.
But once I'd added it, the 20% left felt worthwhile as a means of e.g. do complex searches, or generate tables or otherwise do search and replace of content that requires more (e.g. parsing timestamps and replacing them with another format....) and any number of things I don't do very often but that feels very comfortable when you do need it. To be clear it's not like it does much you can't do easily without it. E.g. after all I could just dump the buffer to a file, load it in my repl of choice, manipulate it and write it out again and reload it in my editor. It's one of those small things that feels unimportant when you don't have it there, that doesn't save you a huge amount of time, but that just makes things feel nicer when you get used to them.