Part of the point was to hand out copies to wealthy allies as a visible sign of favor. Physical arches are a little bit harder to dole out that way, or distribute around the nation. It was also cutting edge technology in a way simple arches weren't.

I guess part of the point is that it's not subject to the constraints of what you could actually do, so you can make it even more impressive. It's propaganda, after all.

Probably. But you would only have one of those in your empire and your populace wasn't super mobile back then, so the audience would be limited.

This on the other hand, you could put up all over the place.