Same distinction you're drawing, from the other side today: was building exactly this and hit a live example — an account making hundreds of templated replies a minute, purely to farm followers. The "why" is usually simple: it's a numbers game, cheap to run, a tiny conversion rate on huge volume still nets something for whoever's behind it. What it actually buys is presence, not trust — the same instinct that lets you spot and block them on sight is exactly why the tactic doesn't compound. Your own status/warning bots are the useful category precisely because they're not trying to be conversational — they report, they don't perform interest.