Seconding @timdenee, since Blades is a “fiction first” game, the mechanical benefit is determined by the in-story capabilities of your rooks.
As @Wildpark said, rooks are “Con artists, spies, and socialites” – but that’s a broad overview of possibilities. They shouldn’t just be a board game token you drop on your turf to make 1 coin per downtime. If you define them and flesh them out, it should become clear what they can do.
In a game I’ve been running, the smuggler crew decided that they’d grown powerful enough that they needed a way to keep tabs on their turf, so they “adopted” a gang of street kids as rooks. Just that much tells us a lot about what they’re capable of (great at blending in to spy on rival gangs or warn of trouble in the neighborhood, bad at fighting with anyone besides another gang of kids, etc…)
The crew has used them to help prep for scores (gathering info or even setting up distractions for bonus dice on the engagement roll), to work on long-term projects (spying out rivals’ activities), to send messages via trusted hands, and so on.
If your crew’s rooks are a gang of neighborhood dealers, I imagine some of the same possibilities exist. They could also send them as proxies to sell the crew’s wares if pcs are otherwise occupied, help keep the neighborhood happy and docile, or to make contact with some new buyers. If you take edges and flaws, and come up with some names and personalities, it just gets clearer.
Really, like most things in Blades, they can do whatever makes sense in the fiction, and it’s up to the table to figure out how to represent that mechanically.
I’d also echo @BlueNinja – the claims for “secondary businesses” are a great way to deal with income from non-score related stuff. They’re heat influenced, which is fun, and they make excellent targets for enemies.