E.g. take one case, recurring appointments. You can do the usual (monthly, weekly, etc.) or embed a snippet of Lisp code in your org-mode file; if it evaluates true when your agenda is being generated, then the item is deemed to be repeating that day.

So, let's say you want your calendar to contain an event if the weather is forecast to be over 40 degrees C on that particular day[1]. Easy if you're using org-mode, but basically impossible with every other system I've seen.

The point behind using a programmable editor is that their programmability allows you to extend them in ways that their original creators never intended. Whereas, say, a conventional email client is designed to be used in just the way its creators intended, and never extended.

I gave a presentation on this topic (the power of programmable UIs) a while back:

https://github.com/duncan-bayne/presentations/blob/master/pr...

Edited to add: in fact, lack of programmability is basically what's broken about modern computing environments. People are so used to software being non-programmable that they don't know what they're missing. It's no accident that even rudimentary programmable environments like Excel are so popular.

[1] ... which is a reasonable requirement for me; the work done by the systems I help to build is highly dependant upon weather.