As for why such accounts do this, we're delving into the realm of illicit activities here on Steemit and it's hard to say with this issue. My guess is one of the following 2:
Get a large number of followers and a large mvest count, then get post upvotes from for low-quality posts. You make at least 20-30 cents per post per account every 12 hours, under current guidelines.
Set up a resteem bot business using these accounts. You can even lump these accounts together and claim to have a larger potential audience reach, despite there being a large amount of overlap between the followers of such accounts.