Why pinstriping on a car? Why art work on a skateboard? Branding, ornamentation, elaboration, beautification, ???. It caught the eyes, increased attention, told people "this bike is special".

A lot (most?) came from the "factory" on top of the line or special order/edition bikes. It does increase value and the components themselves have good value assuming they are actually from the period.

Some collectors (like me) kind of focus on it. But obviously it does nothing for the ride of the bike.

By questioning you have forced me to unleash a torrent of pictures.