"Mind wandering" conjures up an image of random, accidental, and aimless thought fragments going hither and yon like a drunken sailor.  My perspective is much more like Smallwood and Schooler (2006), in which they describe mind wandering as a “goal-driven process”. A lot of mind wandering does seem to be on a mission of sorts: rehearsing, planning, rehashing – as if trying to achieve resolution to some sort of unfinished business. Unfinished business implies a goal – something has not been achieved. Of course, many of these mental missions are aborted mid-stream, as life and other missions intervene.

Reference: Smallwood, J.,&Schooler, J.W. (2006). The restless mind. Psychological Bulletin, 132, 946–58.