Teamfight engage and back-line burst damage.
Burst damage and target access
Hitting a champion with 3 separate attacks or abilities in 2s deals bonus adaptive damage.
Gain a burst of Lethality and Magic Penetration after using a dash, leap, blink, teleport, or on leaving stealth.
Collect eyeballs for champion and ward takedowns. Gain permanent AD or AP, Adaptive for each eyeball plus bonus upon collection completion.
Unique takedowns grant permanent Active Item CDR (includes Trinkets).
+65 health and +9 AP or +5.4 AD, adaptive
After taking damage from an enemy champion, the next 3 spells or attacks you receive deal less damage.
Start the game with extra health. After 4 takedowns, consume that health for adaptive AD or AP.
Electrocute is relative easy to proc on Rakan with a [W]->[Q]->Protobelt granting you a massive amount of burst damage.
Rakan can make pretty great use of Sudden Impact's empowered damage with the use of [W]. The extra Magic Pen will empower your follow-up abilities while Lethality buffs your high base AD auto attacks.
In a mid-game teamfight, Rakan can pick up a large number of kills/assist allows you to snowball those into more burst damage potential.
Ingenious Hunter's extra bit of item CDR is largely used to empower your ability to wave-clear with Protobelt on cooldown.
As Rakan you will be diving in and exposing yourself to a lot of damage without a reliable escape. The extra bit of mitigation will help you survive incoming burst.
Chrysalis allows you to take relatively aggressive trades early if you're able to land [Q] thanks to the extra health.