Had we done this more recently, we might have approached it differently. In a sense, we did, with the digital version.
I don’t exactly recall the original reasoning that we used for this construction, but one thing to note is that the CD4017 only outputs up to about 8 mA per pin. With the transistors, you can get it quite a bit brighter. It’s not particularly uniform in the intensity profile, but it was (to us at least) an improvement over not using the low-pass circuit.
