The Will of all the items scale with Eye of Ekron and Deadshot's to provide Harley with good attack and power damage, as well as boosting the attack /move speed effect of her passive, allowing for even more frequent and high damaging basic attacks.
The attack and power life-steal auras from Atlantis and Qward Gambit respectively make up for the fact that Harley only has a single target heal, in comparison to other Controllers like Zatanna who has a single-target shield that can also group heal and provides power armor with her passive, or like Poison Ivy who have a single-target shield and group heal from her passive. With both auras active all allies, regardless of what type of damage they are doing, will heal so long as they are near you.
The Active from Atlantis Gambit makes for easier secure of kills or escape from an enemy and goes well with her passive attack speed increase and Deadshot's to allow her to very quickly regain HP and Will with basic attacks.
The extra HP from Batman's Utility Belt helps you survive an enemy's combo before healing yourself back up, while the extra attack boosts your basic attacks, the cooldown reduction enhances your skills and the extra spike from cycling your skills to keep your passive active boosts your overall damage.
The Resonant Mod on Qward Gambit helps boost Harley's power damage and allows her to heal for more even with her lower power damage.