Newbies are commenting under the posts of people like you in hope of some rewards that would be impossible to get otherwise. But a lot of spammers are also doing their rounds by repeating 'Nice' and 'Great' comments. The main reason I could find is that they are all struggling with understanding the way of working of steemit. Commenting itself is an art because it requires the comprehension of what the author intended to say and the creation of a good writing. Let the good comments flourish!