> I don’t understand what the “one-off app” is trying to achieve.

Many years ago in college I worked on building Java applets that let kids visualize math related concepts. Sliders make things like sine/cosine and all sorts of other cool stuff way way more intuitive. We had a applet that, let you do ridiculous comparisons, to visualize how many empire state buildings a marathon is in length, etc. We had an primitive 'engine' simulator that let you adjust inputs on a steam engine. stuff like that