Health stacking and map control.
Durability and crowd control
After immobilizing an enemy champion gain defenses and later deal a burst of adaptive damage around you.
Impairing the movement of an enemy champion marks them. Your allies heal when attacking champion's you've marked.
After 10 minutes gain 8 Armor and 8 Magic Resist and increase your total Armor and Magic Resist by 5%.
Every 8 monsters or enemy minions that die near you permanently increases your max health by 0.2%.
15-135 health (at levels 1-18) and +9 AP or +5.4 AD, adaptive
Takedowns on enemy wards cause friendly Zombie Wards to sprout from their corpses. In addition, damaging a champion after destroying a ward inflicts bonus magic damage.
Unique champion takedowns grant permanent out of combat MS.
Like Courage of the Colossus, Sejuani can reliably proc it for the bonus defenses and extra bit of damage.
All 4 of Sejuani's abilities can easily proc Font of Life, and because you naturally stack health the on-hit heal adds up to be considerable for fights.
Conditioning's mid-game resists make you an insane front-line tank in teamfights.
Because you naturally itemize stacking a lot of health as Sejuani, Overgrowth doubles-down on that by providing a lot of bonus scaling health based on how much health you've built.
Sweeping Lens and Control Wards are the mainstay of a jungler. Why not benefit by getting extra vision for yourself when you go on a ward-clearing spree.
It's important for a jungler to be able to quickly react and get to a skirmish happening anywhere on the map. Relentless Hunter gives you the Movespeed Speed necessary to get there and win.