Because minified code functions differently from non-minified code. It helps to detect bugs caused by minification. There's also a general principle which states that the application shouldn't be developed in a "debug" mode, because the debug mode might cause the application to execute differently from production.
Minification can also involve changing variable names; sometimes there are dependencies on names that are unknown to the minifier. Also, the removal of whitespace can cause lines to combine in interesting ways if there aren't semicolons between them and if the minifier doesn't automatically add it.
JavaScript minifaction is much more than that. It renames function parameters, does all kinds of fancy tricks to refactor code and so on. AngularJS is one example where minifacation breaks the code if you don't use the "special" minification-safe pattern.