If you have an area you don't understand or grasp, you can get immediate feedback from a teacher. If you take a class on-line and you come to an area you don't grasp, you'd probably just blow it off. I know of one case where two people were up for a promotion at work and one of the people listed themself as having a PhD and based upon that, was hired for the position. When it was found out the PhD was done online, that person was fired.