Okay I am answering this from a male perspective, yet I think that even a guy who is just watching a movie with this could see a few reasons for this, a lot of which have been said, yet I want to add a few more points to them. Let's take an example of a catfight where basically all hell has broken loose (no rules, no holding back, etc). Basically the most extreme example.
1. Breasts are a one of the key features of the female body, and as such damaging them can not only hurt on physically, yet can be a bit of mental humiliation as well, especially if said boobs are really marked up. I do think an intimidation factor does come into play when one opponent has bigger boobs as more often than not they are the ones that are perceived as being the sexier one, though that might backfire as she'd probably end up with the most damage. In stories I've seen it where destroyed boobs have sent a message not only to the opponent of don't challenge me again, yet also other potential opponents stating that this "woman will go to war with you".
2. Breasts can be attacked a variety of ways to make them hurt (fist right to them, clawing, pinching, biting, kicks, etc.). Also I have heard that depending on the attack boobs can be made to feel a lot of pain (sometimes one good attack is enough to end a match depending on the victims resolve). Like most body parts you probably can train them to feel less pain though, and you can strengthen them.
3.Breasts are opportune targets, as they are right near the center of the body, an opponent can easily get to them. Also stripping the opponent to get to them adds a bit of a humiliation factor, that is until it becomes an even playing field(both topless, or both naked).
4. Breasts are pleasure sensors, which in some types of matches can be used to make one orgasm (conceivably for a win in some types of matches), or at least confuse an opponent (make them let down their guard).
