Math to play with. It has no particular purpose. It is just nice.
Ordinary people keep cats, dogs, fish or dwarf hamsters as pets. Programmers keep pet applications. Pet applications may be useless to others; we write them just for the pleasure of creation. If we have to learn a new programming language, we often rewrite one of our pet applications first. Just as ordinary people keep different kinds of pets, programmers have different kinds of pet applications, often more than one at any time.
This is my favorite pet application. I call it WSTAR. It has many variations, the original WSTAR, the Pascal curve and the Nephroid, and now even this pay version that combines them all.
I wrote the earliest version in Basic, while I was in high school. Then I adapted it to almost all computers I encountered and rewrote it in all the programming languages I learned. I wrote it in Basic, Pascal, C, PL1, Algol, Fortran, assembler, and several scripting languages. It worked on ZX Spectrum, Commodore 64, some ancient Atari computer whose name I do not remember, and of course on PCs, and now on Android.
The application is ad free and open-source (link at the bottom of the store page). Licensed under GNU GPL V2.0.