YACC made it possible for many people who were not language experts to make little languages (also called domain-specific languages) to improve their productivity.
And that is exactly what happened in my case: with no prior academic preparation, I used yacc to build a translator for a COBOL-like reporting and OLTP application language called XPL. (That is, I used yacc, lex, and every trick I could find in a couple of the seminal books on compilers that were available in the early 1990s: the 1986 edition of Aho et al. ["the dragon book"] and Schreiner and Friedman's Introduction to Compiler Construction with Unix, which we called "the unicorn book.")
I did this work for a company called Magna Software, now defunct, which has left few online traces of its existence. I've lost my conversancy with language translation and compilers, so I don't know how it's done any more—I doubt that yacc could support the interpret-on-fly behavior that Visual Studio gives me.