The team had to bring Hampton onto the 53 this week or lose him for the rest of the year.
With the team's dependence on the run, Hampton is another insurance policy (mostly with Gore). I expect that the coaches are not sold on Jacobs (or Dixon) as Frank's backup. Now that Hunter is gone, the coaches are rightly reviewing and consequently restructuring the whole RB depth chart. Personally, I've seen Dixon run with more energy and desire in his few carries this year, but he lacks Frank's quick cutting ability. Hampton has the nearly exact size and style as Gore.
If Frank gets hurt, Jacobs and Dixon may get some carries, but Hampton may just go to the head of the class while LMJ would take up much of Hunter's role.