In many cases, having a large number of followers seems to breed even more of them, so "priming the pump" with a healthy follower base may be an easy way to get a new Twitter account some visibility--and eventually one of those legitimate followers might result in a sale. (On the other hand, this could all be wishful thinking: An internal study undertaken by Coca-Cola found that social media buzz had "no quantifiable impact on short-term sales.")
You don't have to be an unscrupulous scammer to play the game. The New York Times recently reported that accounts belonging to Pepsi, Newt Gingrich, and 50 Cent were all stuffed with fake followers, identified as such because they had acquired or lost a large number of followers in one day. Whether the follower boosting was due to business competitiveness, an attempt to manipulate political polling numbers, or simple vanity doesn't really matter. It's cheap, and there are no real penalties if you get caught.
The unlikely solution
The fix for this problem is incredibly easy, if wholly unlikely. All Twitter has to do to make boosting go away is to stop publishing the number of followers a Twitter account has. If third parties can't tell how many followers an account has, the impetus to boost that number largely vanishes.
Then again, maybe that wouldn't matter, either. LinkedIn caps its public display of the number of connections an account has at 500, but the market for fake LinkedIn connections is just as vibrant. Buying a connection to 100 people you didn't work with will currently cost you about $15.