in multiplayer it's a cheap way to scout enemy's build and design your ships to counter it (also it can give you an idea if you'll even have a chance against him). after all your branch office will be there for at least a year.My experience is that it's a waste of resources to try and build branches as criminal syndicate.
as for the normal use of offices - try to target planets which already have big population and crime (and if they have a deal with a lord it's even better). ecumenopolices are the best targets.