If you're competent with your language, you don't always need to build a special purpose tool with it to do work - you can use it directly. You picked an example where a) libraries are lacking (high level audio manipulation) and b) good special purpose tools exist, but it's entirely possible to find tasks for which these things aren't true. The whole idea of shell scripts as a basic tool, or IPython/Jupyter as a reasonable interface to a variety of tasks, is based on this concept.

In my last "proper" job, I wrote a lot of code, and the vast majority was one-off; virtually none of it ended up in what you might call an "application".