Sometimes you'll approach a girl, hit it off, things go great, and then she brings up her boyfriend.
Now, if you don't like dealing at all with attached women, you can just hit the exit at that point. If you're more of the "it doesn't really matter to me if she says she has a boyfriend" camp, however, you're going to need a response.
79% of unmarried women are in relationships at any given time. Therefore, unless you're meeting women in venues that select for unattached women (nightlife, dating apps, etc.), you're fairly likely to run into lots of these girls who are already attached.
Assuming you run any day game, transit game, or another style of approaching where you're bound to run into boyfriend-mentions, if you want to seduce these girls, you'll have to address their boyfriends.