It's pretty simple really; a lot of "big" youtubers (e.g. HowToBasic, ChaBoyyHD, PressFart just to name a few) do this kind of attention whoring to either feel good for getting a lot of "likes", or to direct more people to their channel.

The problem is that their comments are usually low-quality and meaningless; maybe a catch-all phrase or meme that can be attributed to almost anything (e.g. "MLG"). It rarely has anything to do with the video other than the title.

This results in REAL, constructive comments (even comments from the video's owner), with actual meaning and sincerity, being buried underneath garbage like this shit; all because the fanboys of said youtubers jerk themselves off at the thought of their favourite youtuber "liking" something that they also like; so "liking" that comment gives them some false sense of self-worth, because they interacted with somebody "big" on youtube.

The reality is that these "big" youtubers find the most popular videos and post a shitty comment for attention (as I stated in my first paragraph); they don't care who comments on it, and they certainly don't read any of the replies they get (inb4), it's all about the attention and publicity.

TL;DR shit's cancer and it needs to die, also Google+ commenting system is utter garbage.