The bridge is a great position for activating the glutes and stretching the hip flexors. Adding alternate knee extensions while holding your hips level engages your core as well.
I like to do this exercise in the middle of a series of crunches and similar abdominal exercises. Although valuable, those exercises strengthen your abdominals in a position of hip flexion. Standing, walking, and running all require a strong and stable core in flexion and extension. It only makes sense that we should be strengthening in both positions. This particular exercise requires that your abdominals engage in relation to a neutral or slightly extended hip.
Stay strong!