There sems to be some confusion (not here, but elsewhere) about why a player would be Designated for Assignment (DFA), and then subsequently be optioned to the minors.
A lot of fans (and media) have gotten into the habit of using the term "Desginated for Assignment" (or "DFA") as short-hand for cutting a player, but that is not exactly the correct use of the term.
It all has to do with the relationship between Optional Assignment Waivers and Designated for Assignment.
Last Friday afternoon about a half-hour prior to the 4 PM (Eastern) non-waiver trade deadline, the Cubs acquired RHSP Dan Haren from the Miami Marlins for two minor league players (RHSP Ivan Pineyro and INF Elliot Soto), and then right at the deadline the Cubs acquired RHRP from the Baltimore Orioles for OF Junior Lake.