Building and maintaining strong core muscles improves your health, posture and sense of well-being. We spend a lot of time, in Bikram yoga in particular, working on the muscles surrounding the spine and on the abdominals.
At every point in the class, I encourage you to focus on holding in your stomach and maintaining correct posture and alignment to get all your muscles working. These poses are especially important in the sequence for improving your core.
Questions? Ask in the comments!