>I would have thought the purpose of bots on twitter was either to boost follower counts or retweet counts

Another idea: Could be that the bots first establish a base-line as "non-spammy", i.e. legit, users by posting by themselves without sending any links directly to people. This goes on for a couple of weeks, until Twitter's automated anti-spam-process classifies them as "legit", after which the bots switch over to "spam-mode" and start spamming. Just an idea.