I would say Python's killer app is its massive library -- the size and quality has few rivals. No doubt Python's readability and approachability contributed to this.
While I love Python, I always worry about any language that resorts to this argument, since the lack of libraries is the easiest problem for a language to fix. I still remember hearing (and believing) that Python would never be as popular as Perl, since Perl had far more libraries.
If you mean its built-in standard library - in some cases, it's just a place for code to rot and stagnate. Ubiquity comes at the cost of massive friction to update.
