They are for making parallel lines, using the donuts that fall out of them (keep them!) as described in this post: https://spirographicart.com/2014/09/06/wild-gears-parallel-lines/

The larger holes make a more rounded version of the same design made with a smaller hole.

I should think that a woven fabric would be easier to draw on than a knit one like a t-shirt.

Thanks for your response. That is useful information to me. I think I will try experimenting.

Yes, actually I’ve done it. That’s an article I haven’t written yet. I haven’t quite finished the experiment. I got fine point fabric markers (Tee Juice brand) at an art shop and drew designs all over a t-shirt. I stretched the t-shirt over a piece of styrofoam insulation and used glass-headed sewing pins to pin the gears to it as best I could. I still need to heat-set the ink, which you do with an iron.