Because for an interpreted language you can use it for interactive development. I had thought different reasons would apply to a compiled language but judging by the other replies obviously not!
Interpreters tend to have repls, which tends to influence how development happens. I'm not stating an absolute law though.
I think you may be misinterpreting my original question. It was asked out of ignorance, it wasn't a challenge. I was hypothesising some kind of self modifying code, or realtime adaptive optimising or something. I was attempting to show you my current level of knowledge so you could ELI7 instead of 5 :)
It's mostly just that the feature is always useful (that's why it's present so often in interpreted langauges), but muuuuuch harder to do for non-interpreted languages. Notably, a subsequent goal for the Nim feature is indeed to add a full-blown REPL to Nim too, I believe. Also, worth to note that some non-interpreted languages do also have REPLs, e.g.: Haskell, OCaml, C++ (! — see CLING).
Theres exceptions the other way, Awk is interpreted but doesn't as far as I'm aware have a REPL, Sed doesn't either. But then both of those have varying implementations, so there could be exceptions to the exceptions.
The very term REPL comes from a language that's been used compiled to native code since 1960s (Lisp)
In fact, "compiled" programs on many platforms require an interpreter to set them up by dynamic code loading (including modifications if needed).
In ELF header, it's called "program interpreter", which gives a path to a program that understands this particular file and can assembly a running image in memory
