It really depends on the situation. If it a single province surrounded by your territories then it is advantageous to just wait it out. Better to let them starve than possibly die trying to root them out.

However, if there are hostile nations on three sides that means they have something to gain from assaulting you. The Jaffa campaign illustrates how it will look. Instead of being randomly placed without the castle to be seen, anytime a siege is attempted to be broken the castle will be on one side of those who are doing the siege.

Example, an army rests inside the city with supplies for a year. Your army is strong enough to deal with the enemy inside should they decide to come out. A year passes and word has gotten around of the siege. The faction you are sieging sends another army plus has rallied an ally to help. So you have them attack the siege forces, and at the same time you sally forth. This leaves the enemy between two great armies and a small force that is still going to be a hastle. How would you deal with that situation?

Whereas if you assault the moment you hit the target you avoid the enemy reinforcements.