Just another way to get more fuel and air into the engine. It really only makes practical sense on stock vespa cases because you're so heavily limited with the stock carb location (13mm on stock cases unless you're willing to do a bunch of a fabrication and modification). With most other platforms you aren't limited like that, so you just put a larger carb on.
But even then you can argue for the same price you may as well go with aftermarket cases. So there is certainly a novelty aspect here. Or run ONLY 1 huge carb on the side inlet.
Another thing to mention is that by running dual carbs in this config in a vespa, retaining the stock rotoary intake, you can get really funky with asymmetrical intake timing and overlaps between the opening and closing of both the rotary inlet and the reed valve. Performance wise, I have no clue what this will mean. If anything, it's just going to make it a bitch to tune lol.