In the case of C#, their previous compiler was built in C/C++ and was apparently very hard to work with. Since rewriting in C# itself, named 'Roslyn', they were able to build it with immutable structures, as well as expose the whole API as a regular C# library. Now they are adding new features all the time and the IDE integration is even better.
It's also really powerful for users to just add the lib, and be able to immediately parse any C# solution with the official tools, get semantic info, errors, autocomplete info etc as well as compile scripts. You can get a mini visual studio up and running in a day.
I can even get the source off github, and continue working in my favourite language. I even did this and changed the parser round (removed need for semicolons, make the language whitespace sensitive) just as a learning exercise. It also gave me a bigger appreciation of functional programming, and how one can use immutable data structures when writing a compiler.
Edit: one of the devs even used it to make real time C# [1] ala Bret Victor style.
[1] http://comealive.io/Tracing-Code-Execution-with-Alive/