Why Disc priest need haste rating?

Why Disc priest need haste rating? If I have big haste rating I will a lot esier spaming my power word shield? sry for bad eng

In 10-man raid setting, you will be using more of your spells that have cast times.
In 25-man raid setting, it's all about reducing the GCD (global cooldown) so that you can cast a shield once every 1 second instead of once every 1.5 seconds. For this environment, you don't need much haste rating though. Just enough to cap your GCD reduction while Borrowed Time is active.

Thanks! But whats about pvp? And how much i need haste rating etc?

Haste in pvp is personal preference, but if you utilize Borrowed Time properly, you should be fine without any.