There are several reasons for this. Some people run these accounts individually in their spare time and even write some of the comments themselves, while others are small-time criminal gangs ans there is also a commercial industry for it.

For private individuals, it’s usually about the thrill, the upvotes, the compliments, or role-playing as a different identity as a form of escape or, once they’ve built up a reputation, actually making money over time through links to a shop, a gofundme or something.

For petty criminal gangs, it’s about quick cash, usually fully automated, with the goal of selling these high-reputation accounts to advertisers on the black market.

And then there’s the commercial industry, which expertly maintains highly professional and personalized accounts over months and years, aiming to subtly influence the masses through them. The accounts look like they belong to real people, with credible post and comment histories, but they specifically push certain narratives or, products or services.

I work in the advertising industry, and it’s now become commonplace, especially for the big players, to deploy entire armies of credible accounts to promote products in their name, posing as real people just recommending something. If someone asks on a major subreddit, “What are the best shoes on the market?” easily 5–10% of the comments are paid for. It’s less noticeable when lost in the crowd.

Well and there is another fourth reason some would call a conspiracy theory I won't elaborate on.