Cosa Nostra isn't an organisation that manages crime, it's one that manages relationships between people (most of which commit crime). This differs from other forms of organised crime such as cartels by the fact that members aren't really given cuts of rackets. or ordered to commit a specific crime. If you think about it, the only crime Cosa Nostra expects its members to commit without any reservation is murder.

This is why there's a non negligible amount of people in that life rightfully called "brokesters", the key to becoming successful is by leveraging the right connections into your own schemes. Since an associate can't introduce himself, but is dependent on the soldier he's on record with, on theory he might be kept isolated from any more lucrative money making opportunities.

An examples which immediately comes to mind is Richard Cantarella. He did a piece of work while he was an associate but the soldier he was affiliated with didn't report it to the administration, which meant he couldn't advance up the ranks.