The first reason I think Ed makes sense is Rex Ryan. Rex was the defensive coordinator in Baltimore during Ed's best years and the two had a great relationship. Rex is on the record of saying that Reed is the best safety of all-time, showing Rex has a lot of faith in him. I know Reed has not contributed this season, but last season in Baltimore he was great, with four interceptions and 58 tackles.
Another reason it makes sense for the Jets is need. Safety is not a very strong position for the Jets. Antonio Allen has emerged as a very nice player, but Dawan Landry has been underwhelming. A guy like Reed would make a quarterback this twice before throwing over the middle, as Reed is a guy who can hit very hard. Ed Reed would bring great veteran leadership to a young defense, and would really help the Jets make a playoff run.