Haste rating increases the rate of your auto melee attacks. For every 32.79 points of haste rating, your attack rate is increased by 1%. Contrary to a common misconception, this does not decrease your swing timer by the hasted percentage but rather increases the frequency of your strikes. Consequently, with 20% haste, you cannot simply take 20% off your swing speed to calculate its effects. Rather, you are performing 20% more swings in the same amount of time, effectively granting you a swing speed reduction of 16.67%. The exact formula for swing speed reduction can be seen in section 8.2.
Unlike for casters, haste rating does not decrease your GCD.
With an increased number of attacks over the same period of time, your rage income arrives faster. This can help with your heroic strike frequency. Not only this, but with an increased number of main hand attacks comes an additional number of opportunities to queue Heroic Strikes. However, please note that this will have an effect only if you are at or near 100% heroic strike frequency. If you are not, your Heroic Strike usage is usually not limited by your strike frequency but rather by your rage income.
Increased attack speed also helps re-proccing your proc-based effects that are subject to internal downs. It can also increase the uptime of procs that are not ICD-limited, even if the proc chance is determined on a PPM-basis which is discussed in section 7.9.1.
