Or when you'd like to be able to DM them. It would need to be coupled with very active following to be a spammer tactic. Don't assume malice when other explanations suffice.
I'm not saying they're trying to spam people, it's just that there are literally tens of thousands of Twitter bots that will automatically follow you back if you follow them first.
The number of followers is meaningless if they're not actual humans who are interested in your tweets.
