monodevelop - Why GTK visual designer uses "pack" system to arrange widget? -

I am a "classical" programmer with "classical" RAD system. I want to "migrate" to deploy some applications, but the first big "mountains" to learn "packing" system for design object. Why gtk (or mono) uses this system?

Thanks

A brief overview of you. The main point is to not use fixed cordinates, but screen sizes help you to be independent.