Because whenever you can add the third best corner on the second to worst pass defense in the league you have to do it? Maybe the Jets are trying to distract the Redskins so they don't throw a ton of money at one of their free agent wide receivers?
I think there are a few possibilities.
A. Buchanon would replace Antonio Cromartie.
B. Buchanon would play nickel while Kyle Wilson would replace Cromartie.
C. Signing Buchanon would have no impact on Cromartie. It would spell the end of Drew Coleman as a guy to push Wilson for the nickel spot.
I can't say this move would excite me terribly. Buchanon is not anything special. If memory serves, he was at his best in Monte Kiffin's zone based scheme, which is not really what the Jets do. He's not terrible, though. If he was fourth on the depth chart behind Darrelle Revis, Cromartie (or some other starter), and Wilson, I would not complain. My money would be on scenario C.