Well, I have never done any right proper experiments for anything, so I will just provide answers based on my personal experience.

You cannot acquire new skills or new knowledge from lucid dreaming. At best you can practice skills that were obtained in waking life to fine tune them and formulate new ideas based on knowledge gained from the real world. That's still incredibly useful. You can use lucid dreaming to become better at old skills. You can use lucid dreaming as a source of inspiration to create something in waking life. Lucid dreaming can also be used to learn about yourself and explore your own mind on a deeper level to potentially work out complicated problems from waking life. Another minor thing it can be used for is stress relief.

That's all of the practical uses I can think of at this time.