would make sense if:
- impacts on the economy of the city (positively or negatively according to the player's will)
- you can build such criminal relationships in any city (owned or not).
-you can recruit criminal units.
and now the important things:
- you can use them as an aid when you escape from a prison or make someone escape from a prison.
- you can use them to attack the besieged from within.
- they can open the city gates to you during a sudden siege (not already in progress).
- they can sabotage the city (if it had something sabotable).
- they can be used to assassinate or injure the lords present in the place. In case of failure the man sent to assassinate is killed or imprisoned and interrogated and in any case whoever commissions the assassination loses influence with the criminal leader who commissioned it and the criminal leader also loses influence towards the player or whoever has commissioned the murder.
If the interrogator speaks, then the criminal leader is arrested if he is still in town.
(so he could move elsewhere).
If he is not in town, his criminal gang is assaulted by the city troops and lords present and dismantled.
Conversely, if the criminal gang wins the fight, they take possession of the city.
This would be a criminal empire, a power capable of subverting certain situations.
Not a trivial variable that only decrees if you make some money more or less.