People normally use paid upvote bots to boost their posts a little and make them a little bit more visible. Nothing wrong with that. On Steem there is already a feature for that -> Promotion. The problem is that nobody really uses it. Mainly because of the way it was designed. Promoted posts on Steemit are only visible in their separated tab. There are a lot of ads and stuff by ICOs. Most people don't want to look at that. The quality of content is lower in general on the promotion tab.
Other platforms had the same problem and they integrated ads and paid posts into the regular content. That way those posts would get more exposure and people would only see ads/paid posts from time to time. Bad design by Steemit in my opinion.
That was one of the driving factors why paid upvote bots came to existence. There was a demand for it by the users. They are willing to pay so that their posts receive more exposure. New users don't have many followers and it's hard for them to get started on Steemit. I had the same problem at the beginning. Paid upvote bots can help you with that. They provide a valid service to their users, so there is no real problem for paid upvote bots in general.