And for songs, often (fake?) testimonies to people's emotional attachments to the song.
Whether that actually increases the value of an account or not, I'm not sure. But it's enough that spammers seem to think that having comments & replies on an account.
It may also be purchased comments to improve "engagement" metrics on videos for the creator.
And lots of the cesspool was not caused by bots.
This was also around the time they removed the thumbs up/down count.
This was to prevent creator burnout; imagine if every video you put out had some snarky diss against you in the comments.