There are still a lot of things I don't understand about this subject. So I hope you don't mind if I ask some questions?
1 Why would anyone let a bot buy votes? This bot could easily lose money right?
2 How will my post gain visibility if I buy an upvote for let's say 0.5-2 Sbd?
Probably for bot-farming.
Automate the process of buying the guaranteed profit vote, get at least the SBD you put in back and also a little bit of SP on each dummy account, operate a huge ring of bots and hope steem/SBD stay about the same price for 7 days, then power down all the accounts, sell the steem and recycle the SBD to buy more votes endlessly. Easy way to farm money for the organizers of the bot-nets, I'd guess.
The amount of available upvote on MB is not infinite and they can be "sold out" if the demand is bigger than the supply of votes. And I think the demand for upvotes greater than 1cent is really big. So instead sending these upvote requests by hand some ppl use bots to spam the requests until they get it.
The posts are shown in trending or hot ranked by their possible reward amount, so bigger upvotes=better ranking and visibility.
Thanks for your answer.
About item 1, I didn't know that you will always have a little profit from minnowbooster. I thought it works the same as all the bots with 2.4 hour bidding rounds. Which could result in a loss when there are to many bids in 1 bidding round.
About item 2, I Happen to have a different vision about this subject. I believe there are so many paid upvote bots, that it doesn't give any extra visibility. Simply because there are 1,000+ Steemonians that use this option on a daily basis.