Since Guerra is out of options, he does have to pass through waivers. As larry pointed out during the discussion of his signing, there probably is little risk that he'll be claimed on the way to Charlotte. This move takes the 40-man roster back to 39.
There are also reports that the bullpen competition is over as well.
White Sox bullpen shaping up as Jones, Lindstrom, Belisario, Cleto, Webb, Downs, Veal. Roster moves expected after game tonight in Brmingham— Daryl Van Schouwen (@CST_soxvan) March 28, 2014
If Webb actually has won the job, the open spot in the 40-man allows the Sox to be a bit more flexible going into the season to scoop any interesting waived players or to add someone later if needed. Also, the bullpen might not be as settled as reported and the open spot could be to add Zach Putnam or David Purcey. We'll see what happens over the weekend.