Walaikumsalam, it does sound scary. But it really isn't. and it is very different from day dreaming and drifting in thought. because your body is sleeping but you are full awake in yourself. I don't think lucid addiction would be bad. There were alot of experiments carried out and its a natural phenomena. In the state basically you can do anything you imagine, fly, jump as high as you can and stuff you can't do in reality. Where do you 'go'? No where, just in sleep, no I have heard that once your asleep your ruh leaves. So with respect to where your ruh is, I do not know. Purpose, I am really not sure about it. But lucid dreaming is mostly recommended to those who have alot of bad dreams