They offer higher sound quality than typical potentiometers by using discrete resistors, which allows manufacturers to use a more ideal resistance and better matching between the channels. The trade-off is that you are left with a fininte number of sound levels possible, often as low as 24, which some people feel is too coarse a control. An alternative is to use an internal resistor network, whether controlled by mechanical relays (Presonus Central Station) or programmable IC's (Lavry DA10), which can offer a greater range of volume control options, while maintaining the higher quality resistors.
A stepped attenuator in most cases consists of two sets of many position switches that connect different resistors for precise control. The switches are very high quality and the resistors are precision parts that are matched. They are expensive to build vs normal volume control.
How much of an amps audio performance and SQ is limited by the pot? Lets take a cmoy for example? Say you were too use a stepped att. in place of a panasonic or other pot... would the difference be audible?
Well I am not sure if you could hear the difference with a CMOY, though it certainly would be foolish to put a $60+ volume control in a ~$10-15 amp. The only time I was able to compare the two in the same amp was with a dynalo, and it was a quite clear and noticeable difference (against an Alps Blue). Then again considering the prices involved, it made sense using a $60 volume control in a $250+ amp. Now you'd need more of an amp to justify the cost of a DACT, unless you just really love the way they feel.