Clearly if they exist they must have some marginal use. It's like saying "who falls for nigerian prince scams?" or "who is influenced by ads?". Clearly some people are, otherwise they wouldn't be quite as popular. I'm sure some real people follow porn bots in order to... see porn.
As for the influence I think in isolation it's probably negligible, but if people get flooded by propaganda on social networks it may be quite efficient. "Everybody says it, it can't be completely wrong". The echo-chamber nature of many social networks probably plays into that too. Many people would probably be more likely to follow an obvious bot spamming propaganda that aligns with their beliefs than a real person holding a different opinion. For instance an obviously biased propaganda outlet like Prager U has 2 million followers on Youtube, I doubt all of those are bots.