The mirror can be helpful for developing your kung fu ability.
Before or after class, you typically have time to practice anything you like: kicks, stances, forms, chin na, san shou, etc. By paying attention in class, you see how higher rank perform certain movements. Perhaps you had a movement that was taught or corrected in class by Sifu. The mirror let’s you judge for yourself just how well your movement stacks up. Is your technique well balanced? Are your stances low and strong? Are your kicks and punches fast and sharp? Is your posture correct or are you leaning, tense or just off? The mirror and your honest judgment will give you the answers.
During class, the mirror is helpful in a different way. Like the above, you can measure how high, fast and powerful your kicks are getting, how your stances compare to the rest of the class, etc. However, during class, the mirror can be used when being taught new movements and greater details of old movements. You can see multiple angles via the reflection and see things you might not have seen otherwise. The mirror can also provide you with better peripheral vision to ensure you don’t hit or get hit by others. It can even help you see others if you get confused or stuck – hopefully that doesn’t happen.
The mirrors can do all of that for you and more – the only thing you need to do is use them properly (i.e. not a great idea to look at yourself in the mirror when sparring and definitely not when you are standing at attention). And one more thing, clean the sweat off of them after class every now and then.