In the past decades I have written a bunch of software. And most was rather ephemeral stuff, like hobby projects or university assignments. In most cases there was no need for any sort of formal documentation. I would understand the code as I am working on it, and once I have abandoned it, it would never be touched again anyway.
There is a rather simple problem that sometimes is asked in job interviews:
There are a couple of programs that have cell-based notebook interfaces. In this example I will use iPython Notebook (now called Jupyter notebooks). The exact same results can be obtained with Wolfram Mathematica.
Say you see this:
During classical electrodynamics I had a bit of a hard time to understand what spherical harmonics are in a graphic way. If you have some intuition for Fourier series, I can give you a graphic way to understand the purpose of spherical harmonics.