Like Courage of the Colossus, Sejuani can reliably proc it for the bonus defenses and extra bit of damage.
WhyFont of Life
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.
WhyConditioning
Conditioning's mid-game resists make you an insane front-line tank in teamfights.
WhyOvergrowth
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.
WhyTriumph
Triumph on Sejuani allows you to fronline for you team in teamfights to absorb a lot of damage, and keep going after you pick up assists.
WhyLegend: Tenacity
Tenacity allows you to stick to your target for repeated autos and help sure an [E] stun.