"Like, Comment and Subscribe" is a phrase often used by YouTubers to exhort their audience to do the three things mentioned. Although it is sometimes phrased differently, or put as more of a suggestion than a command, it is usually put in just those terms, and put rather insistently and regularly.
The YouTube Algorithm is strange and unknowable, but one obvious thing about it, is that it favors user interaction. The more that users add a like to a video, and the more they leave comments, the more the video will be shown to other people. Subscribing is pretty self-explanatory, as subscribing to a YouTube channel means that YouTube will regularly show you that channel's videos. A certain number of subscribers (currently 1000) is necessary to be monetized by YouTube. Presumably, the number of subscribers to a channel might also determine how much that channel's videos are suggested to other users. No one knows exactly how the algorithm works, but "Like, Comment and Subscribe" seems to be a good way to express how to expand it.
There can also be a snowball effect where a single comment on a video will draw in more viewers, who will comment more, adding more viewers, and so on. The difference between a video languishing in obscurity and becoming "viral" might be as minor as a viewer leading a single comment.
So while it might seem like an irritating and annoying shill, "Like, Comment and Subscribe" exists for a reason, and if you want to support a creator, doing those three things is a good way to do it.