Basically, each Legion had an Insignia, a symbol. For example E Company, featured on HBO television show Band of Brothers, the 506th Infantry Regiment, had an Insignia stitched onto their uniforms. Similarly the Romans had their shields as a means of identification for which legion they were a part of, this might even be the origin of the tradition for each regiment or division of a larger entity having its own symbol of identification.

So the "Painting" didn't really mean anything, each legion had its own symbol and the members of each legion would paint said symbol or have them displayed on their shields. So a very small percentage maybe even less than 1% of Roman shields in this style actually had this painted on it.

As for why this one is so prominent, I'm not really sure, but my best guess would be that the best specimen that we had for a Roman shield had this insignia painted on it so people copied that on their representations of Roman shields, and over time in large part due to the internet this became the go to idea of what a Roman shield should look like.