I love the stat break points in WoW. The idea is that at certain values of a stat like critical strike rating or haste, your character's power improves dramatically due to some complex interaction. Not ever spell scales linearly, and so as you reach certain thresholds, your potency can vary wildly. Holy Radiance's interaction with haste rating is a perfect example. The haste rating we acquire obviously lowers the spell's base cast time, which is how every spell works.
However, at certain values of haste, we can actually cause Holy Radiance's AoE HoT to proc for an additional tick of healing. The extra tick of healing tacks on an extra 10% to 15% healing to each Holy Radiance we cast, dramatically affecting the spell's potency. The problem is that while the first haste break point for HR is easily achievable at 777 haste rating, the next break point requires nearly 3,500 haste rating to achieve.