Although regular nuclear weapons could be intercepted only on paper, no one has tried to do it with real weapons that the other side would use yet, potentially it's believed that it could happen. So the goal is to have faster nuclear weapons that would have less chances to be intercepted. Though, I have doubt it was the incentive in this case. If you watched Russian news, the only thing that gives at least some positive response among people are the achievements of the military industry. Official media milk this theme by producing one note per week (sometimes more) in regard of "promising" weapons that "have no counterparts in the west/world". Most of modern weapons are just soviet developments, so they need at least one real wunderwaffe to demonstrate that last 20 years were not for nothing.