1)appearance. it cleans up the look of your tank so you dont have a bunch bulky PHs sitting in your display detracting from your tanks beauty.
2)flow. adding more flow, and lots of it.
3)efficient flow. less energy consumption with one powerful in line pump as opposed to 5-6 plugged in PHs
4) less heat transfer. inline pumps transfer virtually no heat to the system. when compared to PH's which sit in the water and warm it.
5) less risky. cheap PH's are just that, cheap. they have been known to degrade and even catch on fire underwater! nothing like a cloudy black oil slick in your reef tank. its like your own personal exxon valdez. it does happen.