Articuno's type is good offensively, and while some would want a coverage move, Articuno can manage since Ice/Flying cover each other decently, and Roost is just too good to pass up. With something as bulky as Articuno, that is. But, Articuno's defensive type is quite poor, and while its good offensively, its base Special Attack is average at best, and base 85 Speed leaves even more to be desired. A moveset with Agility lets it pump all its EVs into its bulk and fully maximize its Special Attack with a Modest Nature, thereby softening up its offensive flaw and capitalizing on its bulk. This set does really well as a Special Tank because of its sheer raw stats and access to Roost, as most Special Attackers are unable to 2HKO it without a super effective move; without it Articuno can just tank them to death at its pleasure. Agility is great in general since it gives a good feel and sweeping potential and makes its weakness a lot more manageable (Your opponent has to eat one of your STABs before they can hurt.
