I can't speak to why anyone chooses to use 'bot services', but typically what I've read in post or comments, the reasons break down more or less along these lines:
PROMOTION
People spend STEEM/SBD to get an upvote sufficient to put them at or near the top of trending pages. There's the main trending page, and then there are the trending pages for each of the tags. To get to the top of the main trending page takes a pretty high upvote, and that's probably coming from more than one bidbot or service. The amount necessary to get there is generally out of reach of most members on the platform to do on a regular basis. So, whether or not it's that effective as a promotional tool is debatable.
Unfortunately, users see others doing, see the large value allocated to their post, and think it's something they should do, without really knowing what it means. The actual net value of the post is going to be far less once the amount spent on the votes is factored in, but no one other than the user really sees that part.
BELIEF IN THEIR WORK
This one is used with self-upvoting, too, but basically, it's how some people say they show that they think their post is really good, important, newsworthy, etc. So, they use the bidbots to show that belief in their work.
ROI
There is a belief that using bidbots can give one a fairly reasonable rate of return. In some cases it can, but whether or not that is a universal experience, or something that can be sustained over multiple uses is again, something that is debatable. It requires knowing what bots to use and when, and doing that research every single time. It requires timing and knowledge of how each bot works. And because the bots are not in business to lose money, they will do what is best for business, and that's not always going to favor the user, though it certainly can in cases.
DON'T KNOW ANY BETTER
I believe this is as real a reason as any as to why people use bot services. They really don't know what they're getting into. They see others doing it, so they try it. They may have some success, so they keep trying. And since they're not really keeping track of what's going on, they don't always know immediately if they're actually gaining anything from it.
STEEM has a significant amount of churn. So, it's natural that new people will continue to repeat what they see others doing until they figure out their best path. Maybe that does include bidbots. Regardless, until they get up to speed on how the platform works, they're bound to copy what they think successful people are doing.