Minifying alone is "kind of" important, and the benefits are hard to measure precisely with the combination of gzip, http/2, and the plethora of other network optimizations. However, the more sophisticated JavaScript compilers, like Google's Closure Compiler (https://developers.google.com/closure/compiler/) also do a lot of other compiler optimizations, like inlining functions, eliminating dead code, etc.

When you are working on a large engineering team with lots of JavaScript volume written by a lot of different people, the dead code elimination alone is worth it; it means you can use one function from a library with millions of lines and your compiled JavaScript will only contain that individual function rather than many megabytes of unused code.