Incremental recompilation is a big one, but beyond that, out-of-the-box handling of multi-package projects without the hassle of custom scripting or setting up .nix files just right. If all your projects are single package, `cabal repl` should work pretty well. Where stack really shines, and where it's interesting to simply reuse that, is for multi-package repos, which tend to be very common in private company Haskell developments.