For those who care about their image, authenticity is an important social media currency. See, e.g.: https://www.twitteraudit.com/
For some, it's about the same reason one would report spam: a dislike of abuse.
I feel both of those a little, but for me it's mostly about having a real connection with my audience. I definitely pay attention to who's following me, who favorites my stuff, et cetera. When I write on some social medium, I write for my audience. Writing for a bunch of spambots, marketroids, and scammers is... offputting.
Twitter requires auth for every API call, even reading public information. They would burn out their own API quota pretty fast if they didn't have each user grant them a read-only token.
My main reason is that I don't want to write to an audience of bots. I'm using twitter semi-anonymously so none of my real life friends follow me there (I don't have a separate account for this), so people who follow me and I follow are the people I'd talked to before on twitter. If I'm followed by more bots than actual people, I feel like the social feeling of twitter would fade away. I have a small number of followers (around 70) and keeping my followers as authentic as possible gives me the feeling that people care about what I write. (They probably don't but blocking bots gives me the illusion that they do, because my follow count rarely changes.)
Additionally, I'm obsessively organised and spend a lot of time thinking about the tiniest of details and this may be a product of that tiring (and somewhat problematic) personality trait. This is the same reason why I delete my tweets the day after I tweeted them.
I have a very similar attitude towards bot accounts. Early on, they were instantly recognizable and took no time to identify and block. Now that I'm doing it for a while, I feel obliged to continue and I've discovered that some bot accounts look semi-legit at first (Twitter's 'uncanny valley') but that dragging their profile image into Google's reverse image search would give them away. Same image, loads of different accounts all almost normal but when viewed as a group they were obvious fakes. Curating your Twitter followers list is just a matter of personal preference, nothing more.
And I'm definitely a 'tweet deleter' after reading this article[0] and agreeing with the premise that Twitter messages aren't that important to preserve. I'd rather deny data miners the ability to better profile me than keep old messages. The article's linked-to Ruby script to archive & delete messages stopped working recently but I found a newer one[1] which works very well. Set it up as a cron job, set the preferred threshold for RTs, favorites and time, and it keeps your feed tidy. If that old message was so great your followers will mark it as such and the script will preserve it. The rest disappear after X days.
That's my reasoning behind deleting tweets as well. They just aren't worth preserving. I only tweet about the things I'm passionate about, which are human rights, privacy and politics so I can and do tweet impulsively to vent from time to time. I almost always regret those tweets and I don't want them be on my timeline permanently.
Data mining is a concern, though not a big one because I'm pretty sure twitter keeps my deleted tweets anyway. If I don't want to say something I don't want to be found on the internet, I just don't say it.
I don't tweet a lot, so I've been deleting them manually. Which isn't a big hassle for me. I wanted to use scripts to automate the process but they all require API tokens and twitter doesn't let you have them without giving them your phone number, so I decided not to use them. I try to keep my account as anonymous as possible. If I had been tweeting a lot more, I would've definitely used them but currently they're not for me. Thanks for sharing it though. I'll bookmark it just in case I need it in the future.
I'm fortunate to still have an API token from before they required that :-) Can't make any new ones though. But for the occasional script it can be useful :-)
As someone who has managed accounts for business, keeping an authentic follower list to me is about providing a valuable, ethical service to clients. It would be easy for a client to hire me to increase their followers and for me to do so with the thousands of fake accounts. Instead I choose to provide a true follower increase, meaning people that actually value their product. In most cases business owners and managers are appreciative to this approach. It is better to have 50 followers who engage with your product/service, than to have 1000 followers who do not.
Somewhat related, but my follower count stays about the same on average. I go through spurts where I post a lot and then don't post at all. I don't use Twitter that often for direct engagement with individuals, but do from time to time. About a month ago I woke up one day with over a thousand new followers. I thought for a second I had tweeted out something great the night before and was curious what it was that drew so many new eyes. A quick glance at my followers though revealed that ever last one of the new followers was a spam account. To me this devalues the entire ecosystem at Twitter. Twitter has this hard limit on following 2000 people until you reach some undisclosed threshold of followers (presumably close to 2000 followers). I believe that many of the spam accounts are created solely for the purpose of following people to bypass that arbitrary limit. To me though, Twitter has it completely wrong here. It would seem they are encouraging spam accounts by having this limit in place in the first place. There are better ways to detect spam accounts than saying if a user is following >2000 people yet doesn't have 2000 followers they must be spam. Twitter can tell if a user interacts with the systems and tweets on a fairly regular basis. This to me is a better determination of whether an account is spam than simply the number of followers an account has.
