Example: I'm using Coffeescript/Closure/whatever as well as LESS/SASS/Stylus as well as some javascript package system. Whenever I change any of those files, I want the appropriate compiler to get run. I might want to then cat all of the output files into a single big file (e.g. main.css). Now I can edit-refresh in peace.
But now I'm done developing, I want to test and deploy. Now I really want that minification. Perhaps I want require.js's optimzation binary to run. Anything that wasn't cat'ed together should get cat'ed now.
Sure, you can make this two separate tools, but I'd rather have just one.