I've done it to try and choke every last penny from someone I'm trying to undermine w/o my military.
Like, if I'd prefer the Dregin to quit their war with the Arceans, but I didn't feel like bribing them for a 1 time peace deal, I'd just pull the plug from the trade routes to them and try to sell any 'useless' tech to them to drain their coffers there. Then I'd drown them in spies. The more money I can keep from hitting their factories, the less ships they make, the shorter their war.
Moreso, I use it as a 'cold shoulder' towards someone I don't really like.
