Answering your questions in the more general "DSP" (Digital Sound Processor) sense rather than specifically this product; Advanced tuning capability. Because the xovers built into amps are limited. You are stuck with a specific (fixed) slope with a specific bandwidth of xover frequencies to select from and specific xover types you can use. These limitations are for the most part eliminated with a good DSP. And a DSP does a hell of a lot more than just function as a crossover. They also offer advance EQ and time alignment among other features. Generally no. Your headunit might have some EQ, some time alignment and some flexibility in xover selection. Your amp might have a crossover. But generally speaking a good DSP will offer more flexibility and more options within those features. More xover slopes, wider range of xover frequency selection, ability to select different xover types for different channels, more advanced EQ features and options, more detailed time alignment, etc etc. Also there are now going to be several options for those who need 8 channels of processing in the DSP market whereas there are only 2 headunits I know of that have ever been produced that have more than 6 channels of processing. Many of the external units offer 5.1 or 7.1 surround sound, which few if any headunits offer. This is just a general overview, not an all-encompassing comparison. But the moral of the story is more flexibility, more options and more adjust-ability which results in the ability to obtain a better sounding system as a result. Are these units for everybody? No. Not everybody needs the advanced features an external DSP will offer. But for someone looking to greatly improve the performance of their stereo (primarily on the sound quality side of the spectrum) they offer a large amount of benefits.