the thing about computer art is that you can build systems that generate variations on a theme. what that theme is depends entirely on your creativity. procedural generation is basically controlled randomness — systems that produce endless outputs while still maintaining structure.
this is most common in games where you want randomness without complete chaos: maps, quests, encounters, item generation, terrain, music, or simulation systems. but abstract art is interesting because there are almost no rules at all.
bit operators, strange math, current time, sensors, mouse movement, MIDI, audio data, physics systems, glitches, feedback loops — anything that produces data can influence the output of the system.
sometimes the digital output is only the first stage. pen plotting, watercolor paper, ink bleed, printer artifacts, different pens, different paper textures — all of those add another layer of procedural variation in the physical world.