Well, let's play devil advocate. A bit long maybe.

Since several clients or relay(s) have features called WoT (Web of Trust), there are possibilities that this Bot makers want to bypass that protection. How? The bots start by following 'trusted' users first and assuming 'trusted' users will follow them back. The 'trusted' users maybe won't fall for those tricks, but probably some will fall because they just follow back everyone who follow them without fully checking.

Since WoT use "follows of follows" then this bots can slowly join into 'trusted' users network slowly. The bots who successfully join 'trusted' users network will follow another bot to include those who haven't join into 'trusted' users network.

Those bots now can be used for spreading ads, spam, misinformation, or whatever devilish goal of the bot makers if they have succesfully infiltrated 'trusted' network.