Dimel's use is pretty simple.
In the field, he's a blocker and a pass route threat; his primary role as a runner is as a redzone back.
Of his 16 carries, 14 have come inside the redzone. The other 2 where inside the opponent's 30. 10 have come in goal to goal situations. The logic seems to be that the coaching staff trusts him as a tough runner who will stay downhill and get positive yards. This seems to be true as he only has 1 negative yardage carry in these situations. I don't believe that they think Gronk has this skill set as he has very few carries in any situation. My take is that Gronk is faster and a better athlete, but Dimel is a little more compact and a better downhill runner and blocker.