Not a Rangers fan, but just because you're rebuilding, that doesn't mean you fill your roster entirely with kids that may or may not be ready for the NHL.
He'll occupy a spot for a short time while the Rangers continue to develop their picks and prospects. When the kids are ready they get put into a spot in the NHL they can handle. Otherwise they continue to marinate in the lower leagues, playing important roles they could not yet handle at the NHL level.
Seems pretty intuitive. A rebuild doesn't mean "never have anyone on your roster over the age of 13, even if said players aren't ready for the NHL". I would think people would understand that by now, but I guess not.