Tonight in sparring class we experimented with fighting from a lead versus fighting from a side facing stance. A lead is definitely the more common stance, the one you will typically see in a boxing or MMA match. I don’t know that I’ve ever seen a side facing stance in a boxing match (although I’m sure someone has done it) because of how severly it limits your ability to throw a cross. And in MMA, it isn’t super common either but it does crop up when you see people setting up to throw a spinning strike.
So why drill something so unusual? Well, if you don’t try it out, you’ll never know if it works for you. The side stance, on the offensive end, can provide opportunities for a side kick and any number of spinning techniques. On the defensive end, it limits the targets that your opponent has access to. One risk of the side stance is that if your opponent blitzes you, it can be tough to muster an effective defense. Another problem is (at least for me), it is harder to stay fluid with footwork (compared to a lead).
Another good reason to drill the side stance is that there are some good fighters who can and will utilize the side stance as an effective part of their game. If you don’t drill it at all, it will come as a surprise when someone uses it on you effectively.