Haste. It's a really easy to understand concept, right? It makes spells get cast more quickly. The end.
Except that's not quite the end. Haste reduces the casting time of our spells, sure, but there's a limit to how much it can do so. Haste also make our DOTs tick more quickly, meaning there are certain levels of haste where our DOT spells gain an "extra tick."
These factors give different weights to each point of haste, creating weird, nebulous concepts such as haste soft caps and haste plateaus. With a little bit of math, we can pinpoint the exact value of haste where these caps and plateaus are said to exist. But how much weight should we give these numbers? And how should these numbers affect how we gear?