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The combination of theses things made it possible to make fewer mistakes when writing code, thus saving time and improving productivity.  Declarative Languages provide a schematic, or diagram of the pieces of the application, and are more terse than the equivalent in ActionScript or JavaScript.  ActionScript understands classes and interfaces so it can do a better job of enforcing correct usage of APIs.  The verifier catches errors at runtime.  The IDEs can provide better code assistance because the language is more structured.

A presentation from ApacheCon 2015 that explores things deeper is http://events.linuxfoundation.org/sites/events/files/slides/FlexJS_ApacheCon_2015.pdf

Developing an application by using one or more frameworks really amounts to attaching framework components to each other via code you write.  With JavaScript, you can literally attach anything to anything.  You can assign a String to an variable that was expecting an int.  JavaScript has some 'compilers' that try to catch things like that, but once you start using Object or other low-level base classes like EventDispatcher, you can easily cause the compiler to lose track of the actual API parameters involved and not find out until too late.  Sure you can go around and annotate the JavaScript so the 'compilers' can try to help you, but often you may accept code in library form from somebody, and even load that code at runtime.  The verifier tries to catch those kinds of problems.

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