Ymir the Tusk is a melee strength hero whose arsenal of icy spells pack great potential to grant the advantage in a gank. Ymir is both an effective team fight hero and a confident initiator, whose signature Snowball has the unique ability to bring along teammates into the fight. His other skills block off the area immediately surrounding enemy heroes and slow targets, contstricting their movement, and his ultimate, Walrus Punch, is a powerful blow that deals devastating melee damage and a stun and slow that cannot both be blocked by any single item.

Check this out here: DOTA 2 Tusk Preview Guide

Snowball alone can give a successful gank if Tusk brings any nearby teammates with him. He is very dangerous and deadly if not given attention.
He is usually played as a support, although one that can in fact benefit fairly well from statistics and farm.