Why is it important? What even IS 'spiritual advancement' or for that matter, spirit?
Internal power, self divinity, connection with the All as some say... This is spiritual. I find, no matter what path you follow, lack of belief in anything besides the here and now, besides the temporal, is absolutely meaningless. If there is absolutely no point, there is no reason to seek inwardly. Spirituality is important because it deals with things greater than wars, making weapons, creating cooler video games and 3D movies...