Usually in the prototyping stage you don’t have all the nice art to guide the player’s attention. That’s why it’s super useful to a quick and dirty function to make some simple feedback pop up. It could be a little ring, an explosion, some particles, anything really. Or if you’re using gamemaker just: ‘effect_create_above‘.
With this kind of feedback you can quickly draw attention to something and make it feel more response. Say, the player just picked up a power up, collided with an enemy or reached a new level, for all of these you could pop up some feedback. Maybe just some with some different colors or effects. The point is that you make an event clearer for the player (if needed). Over time, you’ll want to replace the feedback with some proper art, some actual explosions or hit-stun animations, so it’s more in line with the game.
For Circles it started out as a simple feedback function that popped up a ring or a circle. And since the whole game is just rings and circles anyway, it stayed all the way. It just got more refined in timing and tween properties.