1) BB has loved this guy forever, as did a lot of SoSH. I'm sure we can dig up some of the old threads where we discussed trading for him in his younger days.
2) He was known as a very versatile back in his day. Running, catching, pass blocking. Given the Oline performance over the last several games, perhaps they see him as a better choice on early downs to provide the option to either run or help with the blocking that we seem to be lacking right now. White has shown flashes, but seems to be more a pass catching back. Bolden has done well but they likely want him back on ST more (punt returner?!). I think SJax's versatility allows for a more dynamic offense on 1st and 2nd downs going forward. He may be a step or 3 slower than a few years ago, but that shouldn't necessarily affect his reads for blitz pickups and the like and will keep the defense from completely selling out on the pass.