Genuine question: what's different about SLIME/CIDER than any IDE? Auto-completion, documentation lookup, REPL, code reloading, unit test integration... these are all in Xcode, IntelliJ, Eclipse, Visual Studio, etc depending on language support.

CIDER can just as easily be built as a plugin to VS Code. (Actually, more easily.) It's just that the people who built it chose Emacs, simply because they personally prefer Emacs.