The power of Skarner comes from his ability to make picks if he is able to find an engage opening. Predator massively buffs your ability to find those windows to get in and hit your target.
Cheap Shot grants you a bit of extra damage while skirmishing and finding a pick to ensure a kill.
Zombie Ward provides a ton of extra vision, especially for junglers who end up clearing a lot of enemy wards and placing their own deep wards.
Ingenious Hunter is key to keeping your Predator on a short cooldown. A few unique kills and you'll be able to find picks with your [R] on cooldown.
Celerity's bonus Movement Speed synergizes well with Skarner's spires for a bit of extra damage as well as buffing his ability to engage with Predator.
Waterwalking grants you massive map mobility to quickly get to a fight or engage a gank. The added mobility and AD is also great for early game skirmishes over neutral objectives like Dragon and Scuttle Crab.