There's two main reasons: efficiency and quality. A Bronze rated PSU is less energy efficient than a Gold rated unit for example. The main difference is in build quality. Generally speaking, the more expensive units such as Superflower/Seasonic etc. use better quality capacitors and components.
The unit you mention is a 600W PSU, but can only output 480W on the 12V rail. The 12V rail is where the most power is needed. Very little runs off the 3.3V and 5V rails in a modern PC (back in the days of the Pentium II & III, the CPU ran off the 5V rail). By comparison, the Superflower HX550W is a 550W PSU that can output 546W on the 12V rail.