To me, he adds an element of explosiveness and quickness that isn't otherwise on the roster at the position. Also, the Patriots are 17-0 in games Lewis has played over the last two years. While that isn't all because of Lewis, it's a reflection, in part, on his impact.
The Patriots have depth at RB. The story thinks Mike Gillislee, James White, Lewis and Rex Burkhead make the team. Brandon Bolden and D.J. Foster are also possibilities. Of course the issue with Lewis is injuries. How the RB roles play out this year will be a huge fantasy question entering 2017.