Two things:
The collars get stuck in the carb bodies easily.
there is a crossover pipe in the exhaust, so it would be difficult to determine one side popping. none the less, if you did a rejet, you need to balance the carbs. Then you can see if one side is leaning out.
To answer your post, though, it guides the needle into the needle jet and keep the mix accurate (well...ahem, as accurate as it is going to be with a vacuum carb). You need it...
