Impairing the movement of an enemy champion marks them. Your allies heal when attacking champion's you've marked.
Spellbook on Tahm Kench allows you to use your summoners a bit more aggressively in the laning phase to secure a kill, and swap to Cleanse/Teleport in the mid to late game to help you team across the map.
Magical Footwear allows you to save 350g that can be spent on an early Locket to enable your mid-game teamfights.
Biscuit Delivery allows you to sustain early lane poke so that you can safely hit the 10min mark wheb Conditioning takes effect.
Approach Velocity allows you to chase down enemies who you slow with Glacial Augment autps and [Q]s for followup damage.
It also helps you to quickly get into position to save a teammate in danger.
Conditioning on Tahm Kench allows you to become an incredibly resilient tank in mid game teamfights so that you're able to save any teammate with [W] who might get caught.
WhyFont of Life
Font of Life's bonus healing for teammates is easily procced from your [Q] and Glacial Augment slow, empowering your teammates massively for sustained healing in teamfights.