Similarly powerful to having the compiler available at runtime, another thing trickling very slowly into mainstream languages.
"But why would you want this?! That sounds dangerous!"
It's quite dangerous, but so is a 700cc dirt bike.
"Can you give me any practical reason you'd want this?"
Aside from the freedom to redefine other folks' code (see: the derogatory term "monkeypatching"), you can leverage runtime namespace access and manipulation to, when loading a "fasl" into a running Lisp image, identify all classes that you care about and then upgrade them to new definitions without restarting the Lisp process.
Most mainstream languages completely punt on "deployment" (PHP [accidentally] aside, where you can joyfully edit production all day, a paradigm too many folks sneer at in the name of cargo cult professionalism), forcing the system operator to restart the process with new binaries (or pycs, whatever, I'm sure you can generalize the point).
A CL system supports that model, of course, but one can also precompile binaries to be loaded into the running system, redefining system behavior without downtime. Binaries, we should note, that don't include the entire Lisp runtime.
It's not that this provides some sort of day-to-day omg ergonomic bonus over the popular Algol families, but that I feel respected as a software author in a Lisp context in a way that I really don't in many other contexts ("nobody needs generics", to pick on some old low-hanging fruit), and the language providing interfaces to itself in itself is a delightfully low-friction cognitive model.
To return to the dirt bike analogy, I could prattle all day about this, but unless you're the kind of person inclined to spend five hours a week learning to enjoy riding dirtbikes, I don't know that I can spill enough ink to convey the fun of managing a few hundred cc's around the track at speed. Countersteering, inertia management, traction, all of these things are going to go over your head if your idea of a fun time with motive power is a Rivian.
